Change conversion operators (operator K()) to return references instead of values

We simply return a reference to the internal memory instead of copying the object.
This should save some copying when using FieldVector/FieldMatrix with non-trivial
types.  Also, it fixes crashes I have experienced with the ADOL-C automatic
differentiation toolbox [0].

I hope that this patch doesn't have any nasty side effects.  I didn't see any
so far.  Comments for this patch had been requested in FS 1449, and there have
been no objections.

[0] I have contacted the ADOL-C maintainer about this and he said the crashes
are not really bugs in ADOL-C, but that it is simply very difficult to write
a good operator-overloading AD program that can handle operator K returning
values.