Add library public interface (!94)

## What is the new feature
Add a public interface for `libfortran-support`. This is related to #9 
## How is it implemented
Add a `fortran_support.F90` collecting all subroutines and functions in the `libfortran-support` library.  
Now one can do `USE fortran_support` to access all public subroutines/functions in the library.

Approved-by: Jonas Jucker <jonas.jucker@env.ethz.ch>
Merged-by: Jonas Jucker <jonas.jucker@env.ethz.ch>
Changelog: feature