Stage PandExo 3 Update

[natashabatalha]

Major changes to the configuration files:

• keyword changes xoff and yoff are now x_offset and y_offset
• niriss background subtraction was previously hard coded to be "off" for NIRISS SOSS. This now must be specified in the strategy config file
• geometries are no longer default and shapes must be specified in the config file
• extinction must be added to config files

Changes between Pandeia 2 -> Pandeia 3:

New MIRI gain value has been added and introduces a very small difference in the calculation (see figure):