Angular air shower development with 2D MCEq

[kotania]

Features added:

• Cascade equation solution as a function of both slant depth X and Hankel frequency mode k
• Conversion of the Hankel-space solutions to the real (angular) space
• Handling of the 2D MCEq databases and matrices
• Muon multiple scattering in 2D

Some comments:

• 2D MCEq functionality can be enabled using the config.enable_2D flag

• The proposed 2D MCEq matrices are defined on the 10 MeV-10 TeV grid and are nominally valid down to the energies of ~50 MeV (found through trial and error). All of the matrices are extended to low energies (<80 GeV) with the UrQMD 3.4 hadronic interaction model; can consider adding DPMJET as another low-energy model option.

• Some of the code related to extracting decay matrices with muon helicity dependence enabled has been written based off of the old(er) version of 1_3_X, which is why extraction of helicity-dependent matrices in 2DShow is not exactly in agreement with the newest 1_3_X (see https://github.com/kotania/MCEq/blob/2DShow/MCEq/data.py#L488 vs https://github.com/kotania/MCEq/blob/461ff75bab3ed70dd34559005976f6fe429fa2bc/MCEq/data.py#L421).

• The length of the Hankel frequency grid is available from mceq_db["common"].attrs["k_dim"], however not all of the functions have access to mceq_db, so extraction of the frequency grid length is kept as len(config.k_grid) for now.