This function registers a privately-defined coordinate
transformation function written in C so that it may be used to
create an IntraMap. An IntraMap is a specialised form of Mapping
which encapsulates the C function so that it may be used like
any other AST Mapping. This allows you to create Mappings that
perform any conceivable coordinate transformation.
Registration of relevant transformation functions is required
before using the astIntraMap constructor function to create an
IntraMap or reading an external representation of an IntraMap
from a Channel.