Crash when starting new calculation in same directory

[cstein]

A rare crash has been found in

https://github.com/cstein/qfitlib/blob/master/src/qfit.F90#L223

when the user starts a new calculation in DALTON with the same not-cleaned scratch directory.

This bug has very low priority right now but should be fixable with

logical :: mepfil_exists
inquire(filename='surface.mep', exists=mepfil_exists)
if (mepfil_exists) then
    ! decide to open and overwrite or something else.
else
    ! open standard with "new"
end if