Fixes the following assertion thrown by TBB:

	Attempt to terminate non-local scheduler instance

This took a significant amount of time to track down due to a number of
factors. If your looking for a quick fix; set your
tbb::task_scheduler_init to use thread local storage.

This is caused by using Python multi-processing (in our case for the
system tests). The process will spawn correctly (Windows), however
Python will helpfully create seperate thread(s). I assume to observe. In
this case the task_scheduler_init handle would then belong to one of
these threads, as the act of importing our Python module would init the
framework and TBB implicitly.

When the tests run and TBBs scheduling is required it will "sign_on" in
tbb's governor.cpp to handle scheduling. This works because the
scheduler_init handle
is process wide. However internally they then use thread local storage
(TLS) for tracking the scheduler.

As the main process goes to destruct it assumes that we must have
a scheduler instance, since at process level we have done the init.

But we cannot see any schedulers since they belong to a different
thread. We have been getting away with this as the TLS returns NULL
which effectively nops the destruction. But the assert correctly points
out were getting this wrong.

Marking each handle to the scheduler as thread local effectively ties
the destruction of the thread to its init status. For most threads this
effectively means we create a unique_ptr which isn't used. But for our
main threads and weird edge case worker threads it keeps the thread
state consistent.

We've used a unique_ptr since we have an API which allows us to change
the number of threads TBB can spin up. However, anyone else facing this
issue would probably be fine with a thread local object allocated on the
stack.

Update logging calls so they work in sphinx v2.3

Add documentation for the thermal neutron back to back functions

Deprecate simpleapi *Dialog functions

Fix TOFTOF reduction script execution

Use cmake generate_mantid_export_header to generate DllConfig.h (Algorithms)

Co-Authored-By: Harrietbrown <49688535+Harrietbrown@users.noreply.github.com>

Replace header guards in tools with pragma once.

They have low to zero usage and at this point are
just a maintenance burden. They do not work in
workbench and are not worth the effort to port.
In the future it would be better to replace input
in workbench with a function that raises a generic
QInputDialog.

Replace header guards in RemoteAlgorithms and RemoteJobManager with pragma once

Replace header guards in TestHelpers and Types with pragma once

Fix Muon and Frequency Domain Analysis GUIs not opening in Plot

updated elemental analysis ref data

Replace header guards in qt/icons and qt/paraview_ext with pragma once

Co-Authored-By: Harrietbrown <49688535+Harrietbrown@users.noreply.github.com>

Replace header guards in Testing with pragma once.

Replace header guards in MantidPlot with pragma once

Fix Plot Type being in context menu on plot with only one line

Add documentation for UserFunctionMD

Replace header guards in Geometry with pragma once

This is being done in stages. I've made this change on the Algorithms project
The change consists of removing the manually created DllConfig.h header and replacing
it with one generated during the build by cmake. The MANTID_ALGORITHMS_DLL macro defined
in DllConfig.had was already partially used in the Algorithms project - so some classes
had the attribute DllExport and others already used MANTID_ALGORITHMS_DLL.
I've made it so all classes now use MANTID_ALGORITHMS_DLL.
The DllExport macro was defined in Kernel\System.h so I've removed this include from a
lot of files and replaced it with MantidAlgorithms\DllConfig.h.

As an aside the client projects that use Algorithms such as AlgorithmsTest were working
without this change so it appears that having DllImport present on a class definition
(which is what MANTID_ALGORITHMS_DLL expands to in client executables) is optional

Co-Authored-By: Harrietbrown <49688535+Harrietbrown@users.noreply.github.com>

Replace header guards in qt/scientific_interfaces with pragma once

Replace header guards in MD and MPI algorithms with pragma once

This commit removes the unneeded header guards in IMDDimension.cpp

MatrixWorkspace::findY

Replace header guards in LiveData with pragma once

Replace header guards in DataObjects with pragma once

Replace header guards in DataHandling with pragma once