Mark optional input/output files in gmx order
Mark files that are not always read, or which are not written if the
user does not ask for them, with ffOPTRD/ffOPTWR instead of using
ffREAD/ffWRITE. With changes in 5.1, files marked with ffREAD are
really mandatory, since we check already during command-line parsing
that the file actually exists (so that we can give a good error
message). This change also improves the understandability of the help.
Fixes #1954.
Change-Id: I6a991786b8cc48a61eedad9f855a93559bb8a5a0