Standardise function names to match geointerface

[rafaqz]

Seeing we need to call the internal methods to run them on non-ArchGDAL geometries (there is no way to dispatch using the GeoInterface.jl methods), it would be nice if they were all identical to GeoInterface.jl syntax. Then LibGEOS and everything else should do the same.

This means AG.area rather than AG.geomarea, AG.astext rather than AG.toWKT, and most unfortunately AG.length instead of AG.geomlength. We can also just keep both functions.

length will be a pain because we will need to use Base.length everywhere else in the package.

See #366