Issue #438: Use class as default for family and reexport common families and formula function

[athowes]

Description

This PR closes #438.

Reexports brms::lognormal, brms::weibull, stats::Gamma and brms::bf.

Changes default to lognormal() away from "lognormal".

I've also altered the test on the recovery of simulated fit_sex parameters to use the right link functions so that they pass. Also on sampling from prior.

I am wondering if lognormal() has different link functions by default than "lognormal". In particular it uses exp() on the SD whereas maybe the string one just uses identity (which would be bad to do, so good we've moved away from it).

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