* initial
* add TransformDirection enum, fix cython compiler issues
* make RPCTransformWarning more generic
* include GDALGCP* in gdal.pxi
* add GCPTransformer
* typo
* move xy and rowcol to TransformerBase
* change existing transform methods to use new *Transformer classes
* normalize transform direction, update docstrings
* update docstrings
* make subsubheadings of Coordinate Transform subheading
* Add new docs transform topic
* add to index
* update docstrings
* update docstring
* Add rasterio.rpc to docs
* update georeferencing.rst
* update transform.srst
* relax isinstance check for RPCTransformer
* catch invalid transform input to get_transformer
* replace None with identity transform so we test the correct ValueError
* update AffineTransformer
* add tests
* rename method to map singleton coordinate to array-like and validate all coordiantes have same dimensions
* update test
* add more tests
* update docstring of GCPTransformer init
* fix copy+paste error
* relax check on xy transformed values due to floating point precision
* remove unused imports
* fix tests and address comments
Co-authored-by: Sean Gillies <sean@mapbox.com>
