Make ph5tostationxml accept lists of requests for a given network

[nick-falco]

Anther improvement I want to make is to have the ph5tostationxml.run_ph5_to_stationxml(sta_xml_obj) method accept a list of request objects for a given network. This will vastly speed up a lot of repetitive POST requests that currently time out. If you are refactoring the ph5tostationxml.py module please keep this in mind.

For example, currently the ObsPy Fed Catalog client makes POST requests, formatted like the example below, that can be hundreds of lines.

level = 'station'
YW 1001 * * <start-time> <end-time>
YW 1002 * * <start-time> <end-time>
YW 1003 * * <start-time> <end-time>
YW 1004 * * <start-time> <end-time>
... etc.

Long requests currently time-out largely because we process each request independently (performing the same work of extracting all stations/channels more than one time). Adding support for lists of requests to the ph5tostationxml.py API will fix this problem since large amounts of data will only have to be read from each requested network one time.