Read `sites` from `sites_csv`

Addresses: https://bugs.launchpad.net/openquake/+bug/1097618

This patch allows `sites` to specified in a CSV file, which is much more convenient for a very large number of sites.

To use this feature, simply specify `sites_csv = some_file.csv` in a job.ini and populate it with `lon,lat` coordinates, one point per line. The sites will be read from the file and are subsequently treated just like the `sites` parameter.