Add regression test for the simple model with an added solar technology

[tsmbland]

This is to test technologies with zero/very low availabilities in some timeslices

See #1199 (comment)

[tsmbland]

To be honest it might be easier just to modify the existing availability profile for wind, rather than adding a whole new process (albeit not very realistic)

[alexdewar]

To be honest it might be easier just to modify the existing availability profile for wind, rather than adding a whole new process

True. Although might we want an example with solar anyway? I imagine it's a fairly common process type, so might be nice to have as an example.

The most important thing is that we have a test for low/zero availability though.

[tsmbland]

@ahawkes Were you suggesting that we should modify the simple model itself to have a solar technology, or to have a second version with solar added (i.e. so we have equivalent models with/without solar)?

[ahawkes]

I think just add solar, rather than create a new case @tsmbland

[tsmbland]

@AlexisST24 Assigning this to you. Looks like you've done most of the work already, but we'll have to wait until we fix the bug with capacity choices, as discussed

At that point I'll walk you through the process of making a pull request and contributing to the code.