Implement fix for inconsistent treatment of littleh in Leauthaud+11 model

[aphearin]

This PR resolves #1061 as follows:

• The new Leauthaud11 has parameter values that assume h=0.72, so that the numerical values in the parameter dictionary can be directly compared to Leauthaud+11.
• The input stellar mass threshold is quoted in h=1, so that the user needs to rescale the stellar mass of their sample before directly comparing parameter values.
• All user-facing functions such as mean_stellar_mass and mean_log_halo_mass accept and return values of mass in h=1 units, for consistency with the rest of the package.
• There is no longer any dependence upon Behroozi10SmHm. Instead, there is a new Leauthaud11SmHm class defined within the same module. This simplifies the complication that Behroozi+10 assumed h=0.7, whereas Leauthaud+11 assumed h=0.72.

Throughout the docstrings, I have made an effort to write copious, very explicit notes about how to convert back and forth between halo and stellar mass.

Thanks to @zaidikumail for the patience. Comments from @zaidikumail in particular would be most welcome.

[zaidikumail]

I reviewed the changes you made @aphearin. Everything looks great now. The little h usage is consistent throughout and thanks for explicitly labeling the different quantities with ...h0p72 or ..h1p0 to denote their little h footings. Thanks for also fixing the errors in the satellite occupation part, for instance, in lines 392-393 where the correctly calculated halo_mass_h0p72 is being used.

[aphearin]

Great, thanks for taking the time to carefully review the changes @zaidikumail (and for raising this issue in the first place). Much appreciated!

FYI - I have one more bug I'd like to fix and then I plan to do a bugfix release 0.8.2, hopefully as soon as next week.