create a struct to hold the Microphysics runtime parameters

[zingale]

This now creates a header extern_type.H with a struct extern_t that holds all of the runtime parameters for Microphysics. The struct is filled on initialization and returned by init_extern_parameters(). This can replace the global version of the runtime parameters.

This also revives the test_parameters unit test to show how to access things.

[psharda]

Looks like we will also need to modify CMakeLists to reflect these changes. @BenWibking ?

[zingale]

tests pass: http://groot.astro.sunysb.edu/Microphysics/test-suite/gfortran/2024-05-07-001/index.html