use hypermean(), hypersd() to refer to treatment_effects()

[wwiecek]

Currently treatment_effects typically returns a list of length two, first element is the hypermean and the second element is hyper-SD. (For more complicated models this may change, but for the typical models we have now this holds.)

Add functions hypermean() and hypersd() which work like treatment_effects()[[1]] and [[2]] respectively, then change the package syntax (code of functions), documentation, examples etc. to make use of these shorthands.

treatment_effects should still work going forward and in some cases it makes more sense to keep it (e.g. if you want to use both [[1]] and [[2]])