metno/snap: Fortran90 and Fimex integration completed, testing framework

This release marks the completion of the Fortran90 rewrite and the full integration of Fimex as I/O library for meteorological data allowing much easier integration of new meteorological data-sources. With a new testing-framework, future release will be more stable.

With this relese, SNAP was also tested to work with the DWD/Icon NWP data as meteorological driver.