Updating parsing functions.

Updating `reduce_columns` and `get_patterns_from_prefixes` functions
of `parsing` module.

`reduce_columns` now raises an exception if the `column_indices`
argument evaluates to `False`. I.e., if an empty list is provided, an
exception is thrown. This was a preventative change, i.e., there was
no known issue due to old behavior.

`get_patterns_from_prefixes` now returns more robust regular expression
patterns. The prefix now must end with digit characters and has explicit
beginning and ending marks ('^$'). This should make the
`--stat-prefixes` option of `dmc.py` more robust.
Updating `dmc.py` script.

Updating the behavior of the `--stat-prefixes` option. If this option is
used, the arguments are now checked to determine if they end with a
period, and if they do not, one is appended. Thus, the user can provide
"pi.net" or "pi.net." as an argument, for example.