Pull out separate functions for reading files in SetupModel.cpp

[ZedThree]

Minimal changes besides pulling out the bits of the code for reading the data files into separate functions. Reduced scope of some local variables, added constexpr auto binary_density_file_magic_number as it's used in a couple of places.

I did also apply clang-format to the new functions using the config file in #397

[ZedThree]

Rebased to resolve conflicts