Helmert: consider that xyzt.t == HUGE_VAL means t_epoch

[rouault]

Currently when doing

echo "2 49 0" | src/cct  +proj=pipeline +ellps=GRS80 +step +proj=cart +step +proj=helmert +x=10 +y=3 +z=1

we get as a result:

         -nan           -nan          -nan           inf

This is due to cct initializing the xyzt.t to HUGE_VAL

[rouault]

Actually digging more, the particular case is needed since in update_parameters() we multiply the differencial values by dt. But when those differencial values are 0 and dt == HUGE_VAL, the multiplication of both result in a NaN.

[kbevers]

I think this looks alright. There's a grey area when doing something like echo 1 2 3 | cct +proj=helmert +x=10 +dx=. Here a t-coordinate is necessary to do the transformation correctly but not supplying it will default to the time-invariant version of the Helmert.
Perhaps it would be better to fail in those cases. Which was probably my intention with the way the code was written initially (although not considering all cases of possible "bad" input data).