Add 'subset' command

[hobu]

For users who might be processing against a read-only instance, it should be convenient to issue a subset command to query the database and select the Attributes and Metrics for a window and write them into a new, local database instance.

silvimetric mydata.tdb subset mysubset.tdb \
    --bounds "([60,60],[100,100])" \
    --attributes "all" \
    --metrics "all"