BFI: add eckhardt filter and optimization option

Closes #32 (closed)

In order to make mHM capable to optimize on BFI, we added BFI as optional output to eval_interface. Also, a new module mo_eckhardt_filter was added to calculate baseflow from discharge time series.

A module mo_io was added to read arrays from files in order to test the Eckhardt filter. This is mainly taken from the Fortran stdlib: https://github.com/fortran-lang/stdlib