LSST DESC Note on Pserv

[drphilmarshall]

@jbkalmbach needs to use Pserv for his SL investigtaions in Twinkles, so can help write a note that a) introduces the software and b) shows it working in practice. Let's write this as an ipynb format Note, in its own branch.

[drphilmarshall]

PS. This note is part of the Twinkles 1 Notes epic

[drphilmarshall]

@jbkalmbach Note that the Note should not reproduce the usage documentation in doc/Usage.md, but instead can demonstrate this usage, explore a bit more what's possible with the code, and do some functional tests. We should refer to the usage notes from the notebook, of course!