Allow CMake-only builds

[eirrgang]

Python setuptools has been more trouble than it is worth and does not succeed at the fundamental task of dependency resolution at build time. After consultation with experts and stakeholders, we need to at least be able to build all three repos with CMake install instructions on the path towards consistency and simplicity.

[eirrgang]

The PR in progress has a few short comings on Blue Waters

bug: toggling the GMXAPI_USER_INSTALL does not invalidate the cached value of
GMXAPI_INSTALL_DIR.

bug: user site-packages directory needs better handling in the auto detection.

cmake ../ -DGMXAPI_USER_INSTALL=ON
-- The C compiler identification is GNU 4.9.3
-- The CXX compiler identification is GNU 4.9.3
-- Cray Programming Environment 2.5.8 C
-- Check for working C compiler: /opt/cray/craype/2.5.8/bin/cc
-- Check for working C compiler: /opt/cray/craype/2.5.8/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Cray Programming Environment 2.5.8 CXX
-- Check for working CXX compiler: /opt/cray/craype/2.5.8/bin/CC
-- Check for working CXX compiler: /opt/cray/craype/2.5.8/bin/CC -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found PythonInterp: /mnt/bwpy/single/usr/bin/python (found version "3.5.4")
-- Found Python interpreter: /mnt/bwpy/single/usr/bin/python
-- Found PythonLibs: /mnt/bwpy/single/usr/lib/libpython3.5.so (found version "3.5.4")
-- Python user site-packages directory is /mnt/bwpy/single/usr/lib/python-exec/python3.5/../../../lib/python3.5/site.py [--user-base] [--user-site]

Without arguments print some useful information
With arguments print the value of USER_BASE and/or USER_SITE separated
by ':'.

Exit codes with --user-base or --user-site:
  0 - user site directory is enabled
  1 - user site directory is disabled by user
  2 - uses site directory is disabled by super user
      or for security reasons
 >2 - unknown error
-- Found PythonLibs: /mnt/bwpy/single/usr/lib/libpython3.5.so
-- Performing Test HAS_CPP14_FLAG
-- Performing Test HAS_CPP14_FLAG - Success
-- pybind11 v2.2.dev0
-- Performing Test HAS_FLTO
-- Performing Test HAS_FLTO - Success
-- LTO enabled
-- Configuring done
-- Generating done
-- Build files have been written to: /u/eot/hays1/gmxapi-dev/gmxapi/build

[eirrgang]

There was a typo causing the broken site-packages directory detection. I don't know how I missed it...

Automatically recalculating the path when an option is toggled does not fit into the CMake paradigm, so I hope that some documentation improvements resolve that potential confusion.