WIP: Add model responses

[KriSun95]

Still tidying up a lot of the response code.

First addition is the atmospheric transmission code from @mstores98 . The new function att_atmosphere loads in the file and extracts the wanted information. The plotting function asset_atm produces the following figure:

which can be found in the repository assets folder.

I (@KriSun95 ) will keep working on this PR.

• Atmospheric code
• Better tracking of information as the functions are being used (now using data classes)
• Version control of code and files
• convertible into FITS products
• Guide and examples on the repository

@mstores98, can I request that the atmospheric file be edited such that:

• The units for the energy field in the header is changed from keV to eV
• The time field is provided with values in the extension similar to the energy field so the time values don't have to be hard coded into the loading function.

[KriSun95]

I've updated the figure

[KriSun95]

@thanasipantazides @mstores98

Good point to merge before this gets much bigger!!

I'll open an Issue to track the changes still to come.