gmxapi sample_restraint 0.0.6

Sample restraint plugin code for GROMACS using gmxapi 0.0.6.

Bug fixes and stability improvements, build system improvements, documentation improvements

Refer to patches of individual source files for ideas on updating custom plugin code.

• ensemblepotential.h
• ensemblepotential.cpp
• export_plugin.cpp
• src/cpp/CMakeLists.txt
• src/pythonmodule/CMakeLists.txt

Refer to https://github.com/kassonlab/sample_restraint , https://github.com/kassonlab/gmxapi and
https://doi.org/10.1093/bioinformatics/bty484 for more details.