assert on Nhalos > 1

[rainwoodman]

We sometimes do feed an empty catalog to halotools (e.g. if the selection is very narrow). I would imagined halotools shall produce an empty result on empty input.

However, this is what we get instead:

  File "/dev/shm/local/lib/python3.7/site-
> packages/halotools/sim_manager/user_supplied_halo_catalog.py", line
> 180, in _parse_constructor_kwargs
>     assert Nhalos > 1
> AssertionError

A quick glance shows there doesn't seem to be a reason why Nhalos has to be > 1 in UserSuppliedHaloCatalog. Could you elaborate the reason we need this assertion just for UserSuppliedHaloCatalog?

Thanks!

[aphearin]

Possibly no reason other than, at that time, it was less work to just put in an assert rather than test an edge-case with no known use-case. But since you have a use-case, I just put this on the v0.7 list of fixes, which I'm shooting to release in December.