Caluclate time offset for other frequencies than daily
Caluclate time offset for other frequencies than daily

Now the previous changes (observation timesteps can differ from
simulation timesteps) also work for other frequencies, by adding the
method get_time_offset. Currently multiples of frequencies (like
2-weekly or 10-minutes) are not jet supported. Frequencies other than
daily still need to be taken care of in the core of the program by Mark
Bakker.

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