Fix issues #258 and #259

[lohedges]

This PR...

1. closes [BUG] Ions should be excluded in AMBER position restraint mask #258 by excluding the standard free ions (Na and Cl) to the AMBER position restraint mask. This logic is triggered when the manual restraint mask exceeds the mask character limit, so we need to fall back on the assumption of standard AMBER atom naming. This will work in the case of a BioSImSpace workflow, e.g. where solvation is performed with BioSimSpace, but not for arbitrary input. (The restraint is unreliable then anyway.)

2. closes [BUG] The __pow__ operator for BioSimSpace.Types._GeneralUnit doesn't handle fractional exponents #259 by adding support for fractional exponents in the BioSimSpace.Types._GeneralUnit.__pow__ operator. I've also re-orded the dimension mask used internally for consistency with sire, which will make it easier to create GeneralUnit types from these masks. (I hadn't used this before.) These update is largely covered by the existing tests but I've added a further test specific to fractional exponents to cover a range of cases, including error handling.

• I confirm that I have merged the latest version of devel into this branch before issuing this pull request (e.g. by running git pull origin devel): [y]
• I confirm that I have permission to release this code under the GPL3 license: [y]

Suggested reviewers:

@chryswoods

[lohedges]

Looks like I just need to update a unit mask in the ABFE restraint code in the Sandpit. Will fix on Monday.

[chryswoods]

Looks great - I like how pow has let you remove special case code elsewhere. Nice use of rounding. I need to remember to port this down into sire too.

Good unit tests :-)

[lohedges]

Thanks. Yes, this simplified a lot of stuff elsewhere so it was worth doing, regardless of whether it will be needed in the long term.

The merge-base changed after approval.