file unit handler

If a program opens several files in different locations of the code choosing an unused file unit becomes complicated. The problem becomes even worse if there is MPI process specific output. In that case the rank needed to determine the file unit which can lead to conflicts in case of too many MPI processes.

An easy solution would be a file unit handler, a function that returns an unused file unit which is then written to a variable. Since fortran 08 there already is one that comes with fortran. Otherwise it would be a short subroutine.

If we decide to use such a subroutine it had to be used for every appearing new file unit since it only checks for file units which are already in use.