oq-engine: Support calculation geometry specified in a `sites_file`

[Lars Butler]

Currently, we support definition of calculation geometry as a region (polygon) or a list of individual points. The latter option is done in the job.ini file by specifying a single line of text containing a list of site coordinates. Example:

sites = 161.3 79.1, 161.2 79.0

Users running large, complex calculations have requested to be able specifying this site list in a text/csv file, rather than a single line of text. If there is a large number of sites the user wishes to consider, a single line of text is unwieldy. A csv file with two columns (lon, lat) would be much easier for the user to handle. This file would be specified with an alternate parameter `sites_file`. This would leave us with 3 mutually exclusive options for specifying geometry:

- region / region_grid_spacing
- sites
- sites_files

[Lars Butler (lars-butler)]

I have decided to implement this in a slightly different way: instead of `sites_file`, I've called the parameter `sites_csv`.

Pull request containing these updates is here: https://github.com/gem/oq-engine/pull/1113