Allow import of 'base_demand' and 'demand_pattern' from GIS files

[angusmcb]

Summary
Hello again :-)

A small feature request, to allow single demands (not demand lists) to be imported with gis files.

This would be completely analogus to the way epanet (and I think WNTR) handles demands in .inp files. This would allow a very easy way of allocating demands for simple use cases.

Additional context
I think this could be easily achieved with a small tweak here.

[kaklise]

Thanks for posting the feature request. I think this would be very useful. Feel free to submit a PR, or we can add this feature in a future release.

[angusmcb]

I can't look at this this month. I will check back in September :-)