Towards v3: new stellar data, interpolation, sqrt law, and more.

[DavoGrant]

Major updates towards the v3 milestone. In this PR we have:

• Incorporated the MPS-ATLAS stellar gird, both set 1 and set 2 (Incorporate the MPS ATLAS library of stellar models #23).
• Updated all stellar grids to have a uniform format, units, and directory structure (Architecture update #29).
• Added the option to select the nearest or trilinearly interpolate the stellar models (Tri-linear interpolation #27).
• Added the sqrt limb-darkening law (add square-root law #28).
• Updated the fitting algo, enabling the option to return uncertainties on the limb-darkening coefficients (Return uncertainty values for the LDCs #24).
• Enable the user to specify a min mu value when fitting (custom mu value limits #22).
• Updated the integrator to Gauss-Legendre approximation.
• Added the option for users to inout custom stellar models (Allow custom stellar model? #21).
• Expanded the tests. Locally tests now check the stellar models and all modes. CI runs the fitting for mocked up stellar models and throughputs.

The docs have been updated with the latest info for installation, supported stellar models, an expanded set of tutorials, and the awesome logo made by @hrwakeford. The citation page will still need to be updated post release.

[hrwakeford]

I have tested out each of the notebooks and they work well.

• tests were performed using MPS1 and Kurucz (1D)
  I made a comparison between the "nearest" and "trilinear" grid selection criteria for M_H=0.01, Teff=5512, logg=4.47, ld_model="mps1"
  finding the following difference in values

• the min mu value has the desired effect but note should be made of the limits of this, for example when using mu values over 0.5 it is recommended to also calculate the uncertainty as this will increase them on the values computed.

Suggestion:
change the default to MPS2 with a link to the paper in the documents
Reason: the most up to date values for stellar abundances and mixing relations.

Before package is complete
This pull request is a major milestone to V3 but a number of things need to be completed before this is packaged and releases.

• Zenodo data archive needs to be updates
• citation file needs to be updated and clear with examples of statements when using various grids
• link needs to be updated on the exo-tic.github.io page to the ExoTiC-LD read the docs.
• testing will need to be completed on the pip installable package and with other packages that have ExoTiC-LD as a dependency