Get rid of `coerce!` DataFrames hack

[ablaom]

At the moment coerce! is a variation of coerce that works in-place for data frames. But to avoid adding DataFrames as a dependency, there is a hack:

ScientificTypes.jl/src/coerce.jl

Line 197 in 10b3323

if is_type(X, :DataFrames, :DataFrame)

Now we have optional dependencies this hack can be removed.