PolyMap tried to provide an iterative inverse by default
if no inverse coefficients were provided. This proved confusing,
and resulted in incorrect answers if the polynomial was not invertible.
Change the default, update the documentation and provide more tests.

Also slightly beef up the test of Frame.convert. Neither is
a very rigorous test since both use a trivial case.

It's not entirely clear it is necessary but it can be handy.
Also it is referred to in AST examples.

MapBox will now throw an exception if maxOutCoord < 0
(as it already did if maxInCoord < 0).

Fixed all Doxygen warnings I could fix
except documenting =default methods.
This leaves one invalid reference to the CardType enum
in attributes_fitsChan.dox and a host of warnings
about deleted methods due to using an outdated version of Doxygen.