Integration testing for sample restraint about gmxapi

[eirrgang]

We now run the sample_restraint tests as part of each gmxapi CI build, but they are not blocking failures and the tests are incomplete. Moreover, a lot of the integration testing is dependent on the relationship between gromacs-gmxapi and sample_restraint, so at the moment, appropriate testing needs to be triggered manually.

[eirrgang]

begun in #143 but not passing yet. Can't make it a condition of success until tagging gromacs-gmxapi 0.0.6 as a stable base version to build against. Reference kassonlab/gromacs-gmxapi#29

[eirrgang]

Proposed test matrices and testing logic

Continuous integration testing should be performed in all three repositories. Different test matrices are appropriate for different combinations of proposed changes and different dependency chains.

Note that new or updated branches are their own CI "target." Pull requests have a target branch according to which branch they are proposing to merge into.

For each repository, all branch targets should test against the master branches and tagged releases (as defined at the time of the branch update). Incompatibilities should either be fixed or clearly documented, and changes suggested for dependent repositories. Development branches may be aware of more recently tagged releases in the other repositories.

For each repository, the devel branch targets should additionally be tested against the devel branches of the other repositories. This will minimize divergent development to a single PR or less.

Dependencies

gromacs-gmxapi does not depend on gmxapi or sample_restraint

gmxapi depends on gromacs-gmxapi

sample_restraint depends on gromacs-gmxapi for functionality, but depends on gmxapi for testing (that's okay) and some API specification (which should eventually move to the gromacs side of things).