expose `get_values_as_in_model(::AbstractVarInfo)`

[penelopeysm]

it is a good idea to have an interface function that is just like getlogjoint etc. so that we don't have to do getacc(varinfo, Val(:ValuesAsInModel)).values or something like that which relies on knowing the exact name of the accumulator that we want

[penelopeysm]

we could also definitely do with a snappier name

[penelopeysm]

personally i would just call it values but maybe the renaming thing has to wait until we have really well and truly excised metadata from varinfo since otherwise it could be easily confused with getting values from the metadata