The new sip build system complies with pep 517 and
requires configuration by a standard pyproject.toml
file rather than running a single command. Along with
these changes the fixed sip.h file has been removed in
favour of a header generated locally if required.

We now support both the sip build systems as this is
required in our current support for multiple OSes. In
the process of adding support some cleanup has been
included:
  - the .sip files in our tree are now full module defs
    rather than the boilerplate being filled in by CMake,
    giving each module more flexibility.
  - the sipwrapper file has been removed in favour of a
    simpler script to simply strip the problematic code
    for C++17 compatability.

Signed-off-by: Jose Borreguero <borreguero@gmail.com>

Signed-off-by: Jose Borreguero <borreguero@gmail.com>

Allows the location of the installed Python packages
to be updated more easily in the future. This is not
so easy while currently bundling MantidPlot/MantidWorkbench
together but once MantidPlot disappears it will allow
modification of the packaged location more easily.

Also introduces a CMake variable to define the tag
used by the dynamic loader to identify the path of the
loading resource. Reduces copy and paste of setting
RPATH entities in the Python framework

Fixes failure of performance test build that only builds MantidPlot.
f1376efd fixed a bug in an if statement
within `qt/python/CMakeLists.txt` that caused this file to start
being processed when only MantidPlot was enabled.

The version of python36-sip in EPEL produces a PyQt4 that segfaults
when an eventFilter is overridden in Python. So as to not introduce
any further custom rpms we build and package a private copy of PyQt4
for Python 3 builds on EL7
Refs #27631

Defines a Python export wrapper for the C++ class
and a python base package to define other parts of the api
in future.