Run FMDA as of arbitrary time

[janmandel]

Replace time now by a time string from a command line argument. Of course RTMA needs to be available for the time including lookback, or already cached.

[janmandel]

Angel @Fergui has already scripts doing that needs to be integrated into wrfxpy

[janmandel]

Needed for #68 to reproduce behavior over a period in the past.