add device<->host transfer mappers (#282)

* implement TransferTo{Host,Device}Mapper

* do not support transfer to same type

* use {to,from}_numpy

* lint fixes

* add comment

* ruff

* disable spurious pylint warning

* add to docs

* more doc fixes

* datawrapper doc

* more doc fixes

* Improve mapper names

* Bring back numpy actx constructor docs

---------

Co-authored-by: Andreas Kloeckner <inform@tiker.net>