Include SNSL metric

[pgris]

Strongly lensed supernovae metric has been added in sims_maf:

• sims_maf/python/lsst/sims/maf/metrics/snSLMetric.py -> script for the metric
• sims_maf/tests/testSNMetrics.py -> unit test for this metric

[yoachim]

Looking at this and the example notebook:

• There always seems to be a seam at RA=0. Is this a bug in the season computation? Pretty sure the observations are smooth across there, so it must be a bug somewhere.
• Should we be using the extra-galactic 5-sigma depth to account for dust?
• The nside gets passed in with the slicePoint, so it can be handy to just compute the pixel area in the run method, then there's no risk of mis-matching the metric and slicer nside.
• It'd be good to set the units on the super__init__. Looks like it's picking up 'z' somehow, but probably supposed to be 'N SLSN'?

[pgris]

Thanks for your comments. Let me try to adress them:

• about the seam at RA=0: I got the 'season' info when requesting to add the 'season' col in the super.init. I thought that it was the "official" way of estimating the season within MAF metrics. I have added my own way of estimating the seasons in the code. I do not see the seam anymore.

• about the extra-galactic m5: the metric is not sensitive to this.

• abount nside and sliePoint: done. The (pixel) area is now included in the run() method

• units has been added in the super.init