Add option to fill gaps in the time series

[chad-earthscope]

Requested by Panayotis:

In the case where are in a mseed file there are gaps or overlaps, the mseed2sac results to many output files, that should following be merged. Could you add the option -y padding, in which we will get one file (all gaps will be filled with zeroes)

[chad-earthscope]

The most likely addition would be to add an option to fill gaps with a specified value for flexibility and user control.

There should also be a maximum gap size to fill to avoid "generating" huge volumes of data.