Fix "double inversions" in pipelines, require a defined forward operation.

"+proj=pipeline +inv +step +urm5 +n=0.5 +inv" now works as expected,
returning the forward operation of urm5. In principle adding more +inv's
should also work, resulting in the forward operation when an even number
of +inv's are present, and the inverse when an odd number of +inv's
are present.

"+proj=pipeline +step +urm5 +n=0.5 +inv" fails at initialization since
no forward operation can be performed. This is a new requirement, but
aligns perfectly with the rest of the library since no operation without
a forward method exists.

0 files changed, 0 insertions, 0 deletions