Adding CTBTO-requested features to FPv9.3.1 GRIB2FLEXPART

[donaldjmorton]

• When compiled for CTBTO (-DCTBTO), insert a CTBTO-specific greeting
• Remove the _fwd.fp and _bwd.fp extensions from the preprocessed met files. For operational purposes, it's best to have them retain the same name as the original metfile
• Given that original GRIB files and preprocessed met files will have the same name, add in feature to insure no writing into the original GRIB file directory.

[donaldjmorton]

The above work has been accomplished.

A tag of FPv9.3.1b was created for this version. Summary of changes:

$ git diff FPv9.3.1a FPv9.3.1b --stat
 flexpart-testing/case_data/met_data/.gitignore |  3 ++
 flexpart_code/GRIB2FLEXPART.F90                | 72 ++++++++++++++++++++++++++
 flexpart_code/convertfields.F90                |  4 +-
 flexpart_code/fpgridcheck.F90                  |  8 +--
 flexpart_code/getfpfields.F90                  | 12 ++---
 5 files changed, 87 insertions(+), 12 deletions(-)

This ticket may now be closed by anybody with such authorisation.