Refactor modifier classes and add docstrings

[matthewfeickert]

PR #1625 added the modifier builder and combine classes, but they might be good candidates for a refactor that allows for a base class.

They also all need docstrings. This needs to happen before v0.7.0 so that we don't have an undocumented API.

Originally posted by @matthewfeickert in #1625 (comment)

[matthewfeickert]

This refactor should also rename mega_mods to builder_data everywhere in tests/test_combined_modifiers.py

$ git grep "mega_mods"
tests/test_combined_modifiers.py:    mega_mods = {
tests/test_combined_modifiers.py:        [('hello', 'histosys'), ('world', 'histosys')], mc, mega_mods
tests/test_combined_modifiers.py:        [('hello', 'histosys'), ('world', 'histosys')], mc, mega_mods, batch_size=4
tests/test_combined_modifiers.py:    mega_mods = {
tests/test_combined_modifiers.py:    hsc = normsys_combined([('hello', 'normsys'), ('world', 'normsys')], mc, mega_mods)
tests/test_combined_modifiers.py:        [('hello', 'normsys'), ('world', 'normsys')], mc, mega_mods, batch_size=4
tests/test_combined_modifiers.py:    mega_mods = {
tests/test_combined_modifiers.py:    hsc = lumi_combined([('lumi', 'lumi')], mc, mega_mods)
tests/test_combined_modifiers.py:    hsc = lumi_combined([('lumi', 'lumi')], mc, mega_mods, batch_size=4)
tests/test_combined_modifiers.py:    mega_mods = {
tests/test_combined_modifiers.py:        mega_mods,
tests/test_combined_modifiers.py:    mega_mods = {
tests/test_combined_modifiers.py:        [('shapesys1', 'shapesys'), ('shapesys2', 'shapesys')], mc, mega_mods
tests/test_combined_modifiers.py:    mega_mods = {
tests/test_combined_modifiers.py:        [('mu1', 'normfactor'), ('mu2', 'normfactor')], mc, mega_mods
tests/test_combined_modifiers.py:        [('mu1', 'normfactor'), ('mu2', 'normfactor')], mc, mega_mods, batch_size=4
tests/test_combined_modifiers.py:    mega_mods = {
tests/test_combined_modifiers.py:        [('syst', 'shapesys'), ('syst_lowstats', 'shapesys')], mc, mega_mods
tests/test_combined_modifiers.py:    mega_mods = {
tests/test_combined_modifiers.py:        [('shapefac1', 'shapefactor'), ('shapefac2', 'shapefactor')], mc, mega_mods
tests/test_combined_modifiers.py:        mega_mods,

[kratsg]

Note that the discussion in #1716 will be relevant for this issue.