Gapfillpkg edits and corrections

[freiburgermsu]

Hello @cshenry !

Here are edits and error corrections to gapfillpkg, which reduced the line count by ~50. The changes include:

1. unused imports and variables are flagged
2. items() was leveraged to consolidate a loop
3. an excessive intermediary variable was removed
4. variable definitions were consolidated
5. redundant !=None was removed since any non-None value will be interpreted as True
6. new_reactions was added as a class attribute for user convenience and accessibility
7. the undefined groups variable is corrected to m, which prevents a KeyError
8. 1/0 boolean logic is replaced for the added/reversed parameters
9. function calls are consolidated
10. in-line or logic and list comprehension are implemented for greater efficiency and conciseness

I can clarify or adjust any edits at your request.

Thank you,
Andrew