Don't export `scitype` method.

Due to a recent change in ScientificTypesBase,jl from version 2.0 to 3.0, the ScientifictypesBase.scitype method is now a two argument method. Packages depending on MLJModelInterface.jl previously depended on the previous one argument version defined in version 2.0 series of ScientifictypesBase package.
A workaround is needed to avoid making MLJModelInterface breaking for its's users. This involves

1. Defining a one argument scitype method in MLJModelInterface. Something in the lines of

scitype(X) = scitype(get_interface_mode(), X)
scitype(::LightInterface, X) = throw(ArgumentError("FullInterface is needed..."))

The corresponding FullInterface method would be implemented at MLJBase/src/Interface/data_utils.jl.

2. exporting the scitype method at MLJModelInterface (Needed as users expect that the scitype method is exported. see #129 )

But implementing the second step also leads to method import clashes with ScientificTypes.scitype at MLJBase. For now a hack at MLJBase/src/MLJBase.jl fixes this issue, but going forward the scitype method shouldn't be exported. Hence users that want to use the scitype method would have to qualify it with the package name e.g. MLJModelinterface.scitype(X).

Start by reading the scitype definitions and exports in MLJModelInterface, then inspect the workaround in MLJBase/src/MLJBase.jl and the corresponding method in MLJBase/src/Interface/data_utils.jl. Confirm that scitype is no longer exported from MLJModelInterface, that qualified use remains available, and that the method import clash is resolved.