Docs/tutorial for JWST MIRI reduction

[mperrin]

• Many portions of the MIRI code are missing doc strings. See review comments on Adding MIRI/MRS to the forward modeling framework #44
• Need to create a tutorial reduction based on the GQ Lup B dat from program 1640.

@mperrin already has a start at reduction and tutorial notebooks for MIRI based on the scripts by @AlexBidot

[jruffio]

Tutorial(s) to include SNR map with forward model and spectral extraction.