Rename `MixedFormat`? about geoformattypes.jl

[rafaqz]

Totally, it's a pretty vague name. Maybe Extended suggests that we know it has both, but actually Mixed is also the default when you don't know the contents besides that it is well known text/binary. Maybe it should be UnknownFormat ? The result is kind of the same as for extended in practice - we would just try using it with GDAL for the required case and see if it works.

[rafaqz]

Ah I think I misunderstood this a little from the title.

Maybe we can keep MixedFormat (which shows that a type can be either format) and split the Mixed trait into Extended and Unknown? Then so we can mark either that it does in fact contain both Geometry and CRS, or that we don't know what it contains.