improve thread safety of UnitFormulaFormatter, PowerUnitsCheck

[lkeegan]

Description

• use a local map to store component values inside UnitFormulaFormatter, PowerUnitsCheck instead of using the static map in SBMLTransforms
• now safe to call these functions from multiple threads

Motivation and Context

partial fix for #232

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Change in documentation

Checklist:

• I have updated all documentation necessary.
• I have checked spelling in (new) comments.

Testing

• Testing is done automatically and codecov shows test coverage
• This cannot be tested automatically

[fbergmann]

good catch, since you are replacing just the mapComponentValues, this will not be more expensive than the existing code.