Switches to using the JSON library for parsing SANS state objects. This
provides numerous advantages:

- We do not need to maintain a custom (de)serializer
- JSON is a documented standard
- We can switch to type hinting in Python 3, currently a significant
amount of CPU time is spend reverifying typed params
- History becomes less brittle and will work from top level algorithms
- Allows new code to be written in a more Pythonic way (e.g. not forced
to use class level variables for them to be serialized)

To avoid polluting the dev-docs with algorithm-specific info and to
avoid storing/updating many image files the Abins development notes
are moved back into Concepts. The fast broadening method is broken
into a separate document to avoid too much wading through plots.

Move deprecation plans to the top of this document to make them more
visible. The broadening section is a little rambling and will probably
remain so until the theory is laid out more formally.

Images could use a few tweaks (more consistent sizing, remove
unnecessary x values) but I don't want to fill the Git history with
small changes to image files.

Figures are included as image files and the code to generate them is
included as a comment. This doesn't seem ideal as they may be revised
and bloat the repo (or become outdated); is there any workflow to
generate images on the fly when building the docs?

The section on interpolation method is a bit long and should perhaps
be spun out into another document.

Updates the HDF5 URL to the new one. Using browsers or wget would
correctly redirect the request to the new URL, whilst sphinx would get a
HTTP 403 instead.

Updating this URL should fix the sphinx warning, restoring the master
incremental and clean builds to stable (hopefully)

Dan's requested fixes

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

Bumps the minimum CMake version to 3.13, this allows us to use
add_link_options on address sanitizers.
At the time of this commit 3.14 is available for RHEL7, and 3.15 for
Ubuntu. Windows requires 3.15 so in an ideal world we would just set our
minimum to that, but as it's not fully available we settle for the
earlier version at the moment.

ClassTypeParameter and the list variant emulated an enumeration from pre
Python 2.6. Since we have converted to the native enum type we can now
drop the custom wrapper, as no instances are using it.

This commit adds new unscripted testing instructions for the new ISIS
Reflectometry interface. This is based on the instructions for the old
version of the interface from the wiki but updated for the differences
in the new interface.

One of the old methods to produce an unexpected exception and an Error report pop-up no longer worked and this has been replaced. Also included is a link to the Training Course Data for the file in the new method.

We have been using DragNDrop packages on macOS for some time
and this just requires `cp` and not the use of `installer`.

Adds instructions to brew unlink the qt4 installation and
perform some extra setup on the developer machine. This is not
the most elegant solution but requires the least amount of code
changes in our CMake.