newexp-desi use specsim for fiberloss calculation

[sbailey]

Update newexp-desi to use specsim for fiberloss calculation

[dkirkby]

Here is a checklist for integrating the new specsim fiberloss capabilities with desisim:

• Finish Efficient simulation of multiple sources in a single exposure specsim#53 and tag new version of specsim.
• Specify focal (x,y) for each fiber in a simulated exposure.
• Specify the ELG + LRG + BGS distributions of (size, shape) to sample from.
• Specify how seeing of each exposure is generated or input.
• Implement script to generate and save fiberloss files (details in Implement script to generate and save tabulated fiberloss arrays #217).
• Implement interpolation in saved fiberloss files.
• Update quickgen to use new specsim batch API for simulating multiple fibers at once.

I have probably forgotten something so feel free to edit this list.

[moustakas]

I believe that all of these outstanding issues have been implemented in a variety of PRs; please reopen if not.