add status_mds (for the mds status) and status for a global status return, based on the "error_status" output of gdat already existing

@lsimons @mmarin @amerle I have added to expose the mds return status to gdat output structure, so we can use it to check data is ok

Since we might have other return status from specific functions, I called it status_mds.

I added a .status to have only one field which decides ala "IMAS" if it's ok: 0 means ok, positive means specific error that can be found in the code, negative means unknown error

Note there is already "error_status" in return of gdat. So I output also the error_status as .status, since we usually call gdat with only 1 output arg