Return NaN coordinate immediately when receiving NaN input

Temporal gridshifts allow [h|v]gridshift operations to be used as step functions
in a pipeline. This is useful in transformations dealing with deformations caused
by earthquakes.

See the included documentation for details.

Originally the code was doing double to int conversions like

    y = (int)(x + 0.5)

which results in rounding when typecasting. In an earlier attempt to
avoid buffer overflows in integer typecasts this was changed to

    y = lround(x + 0.5)

which doesn't give the origial results. We fix that here by instead
doing

    y = lround(x)

It is safe to so as long as x is positive.

Use proj_strtod.h for proj_strtod and proj_atof

Why was fabs applied to ntrips?  And why is it parsed as a float?
Also add documentation for roundtrip.
External rst docs do not match this function.