Consistent dunder main usage

[ka-rocha]

As discussed in #436, the use of dunder main for command line calls to python files is not consistent across the codebase. The changes added in #436 uses the function protocol while other instances may put the check for dunder main at some unspecified location within the file above the code intended to be executed.

Since the functional use is recommended in the python documentation with rational described there, this issue is to the transition all dunder main calls to match for logical consistency (e.g. bin/posydon-run-grid).