Testing improvements and a clarification to the interface

[tjjarvinen]

This PR will solve couple of issues with testing and add combined testing functions that allow testing everything with a one call.

1. non-allocating force call forces! takes in an array where forces are saved. It can be overwritten or added. It was not defined which one it should be. Now addition is defined and also tested.
2. Combination calls energy_forces and energy_forces_virial where not tested with test functions. This has now been updated with new functions test_energy_forces and test energy_forces_virial. The new function include the corresponding individual tests in them, so you should use them over the individual calls.
3. Added relative tolerance keyword for testing functions. This is needed, if there are different implementations to e.g. allocating and non-allocating forces. For consistency the keyword is implemented on all the calls.

With this the original version (v0.1) of AtomsCalculators should be complete and we can start to implement new functionality (#12) to AtomsCalculators.

Couple of notes:

• We need to write more documentation. But this can be postponed untill we have added the new functionality
• The plan is to tag this to v0.1.2 and then have the new features in for v0.2

Could we get couple of fast approvals to get this one done fast, so that we can move forward to new funcitonality @cortner, @mfherbst, @rkurchin @jgreener64

edit. updated 3. point on the descriptions

[jgreener64]

Looks good, I support clarifying the addition over zeroing of forces since I had a case where addition was explicitly required.

[jgreener64]

Incomplete sentence (and below)?