The +t_obs parameter was confusing for users since it effectively
overwrote the observation time in input coordinates. To make it more clear
what is the operation is doing, users are now required to directly specify
the time span for which they wish to apply a given deformation. The parameter
+dt has been added for that purpose. The new parameter is mutually
exclusive with +t_epoch. +dt is used when deformation
for a set amount of time is needed and +t_epoch is used (in
conjunction with the observation time of the input coordinate) when
deformation from a specific epoch to the observation time is needed.

Changed the direction of dt-calculation to follow the same convention as
helmert. Changed from dt = t_c - t_obs to dt = t_obs - t_c, which
effectively reverses the direction of the operation. Existing projstrings
using deformation can simply reverse the direction of the operation to
achieve the same results as before this commit.

The Geographic offsets transformation adds an offset to the geographic longitude,
latitude coordinates, and an offset to the ellipsoidal height.
This method is normally only used when low accuracy is tolerated. It is documented
as coordinate operation method code 9619 (for geographic 2D) and 9660 (for
geographic 3D) in the EPSG dataset.

It can also be used to implement the method Geographic2D with Height Offsets
(code 9618) by noting that the input vertical component is a gravity-related
height and the output vertical component is the ellispoid height (dh being
the geoid undulation).

It can also be used to implement the method Vertical offset (code 9616)

It is used for example to transform:
- from the old Greek geographic 2D CRS to the newer GGRS87 CRS
- from Tokyo + JSLD69 height to WGS 84
- from Baltic 1977 height to Black Sea height

It is also useful to document the implicit zero-offset transformation
we do in pipelines such as

+proj=pipeline +step +inv +proj=longlat +ellps=A
               +step +proj=longlat +ellps=B

that can be explicited as

+proj=pipeline +step +inv +proj=longlat +ellps=A
               +step +proj=geogoffset [+dlon=0 +dlat=0 +dh=0]
               +step +proj=longlat +ellps=B