Several small fixes

[leeping]

In forcefield.py:

• Fix a .ffxml parsing bug in which leading whitespace causes a floating point number to not be recognized.
• The OpenFF unit string is incompatible with evaluated parameters (evaluated parameters are not independently optimized, but are functions of other parameters). As a temporary fix, units have been disabled for evaluated parameters. @j-wags : Your input could be helpful here, would OpenFF ever want evaluated parameters with units?
• Updated the OpenMM interface to incorporate the new method for obtaining names of Force objects while preserving compatibility with OpenMM <7.6.
• Fixed CopyAmoebaVdwParameters such that type parameters and type pair parameters are copied.
• Added the ability to replace parameter tag names with more descriptive ones when building parameter names (such as <Angle .../> in AmoebaOutOfPlaneBendForce

In objective.py:

• Fixed remove micro-iteration folder feature. The previous code never worked.

In studies:

• Changed iAMOEBA hydrogen vdW parameter sigma to 1.0 (for compatibility with OpenMM >= 7.6).

TODO:

• Undo unintended change to make-vdata.py.

[pavankum]

Fixed remove micro-iteration folder feature. The previous code never worked.

It never worked for me either, Trevor suggested this workaround to me using glob to get the folders and this works, it goes here if this feature is still of interest to anyone, https://github.com/leeping/forcebalance/blob/master/src/optimizer.py#L889

            if not self.retain_micro_outputs:
                for microo in glob.glob("**/micro_*", recursive=True):
                    shutil.rmtree(microo)

[pavankum]

Ahh sorry, I see that you already made required changes, please ignore the previous comment.