Bugfixes for obtaining limits for BEP and FEP models

Merge pull request #15 from jzuhone/fix_limits

[BUGFIX] Fix bug in get_acis_limits

v2.3.0: Merge pull request #14 from jzuhone/simplify

Simplify code base, update get_acis_limits function, add lines to validation plots

v2.2.0: Merge pull request #13 from jzuhone/plot_adjustments

Adjustments to plots for readability and more information

v2.1.0: Merge pull request #12 from jzuhone/acisfp_check

Update which provides compatibility with the ACIS focal plane model.

v2.0.0: Update __init__.py

First working version of acis_thermal_check