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

As identified in #1091, Helmert implementation in PROJ 5.0 and 5.1 is confusing.
It happens that by default it used the coordinate_frame convention, contrary to
the position_vector convention used traditionaly for +towgs84. The documentation
of Helmert was also wrongly specifying that the default convention was
position_vector.

This commit:
- bans the confusing +transpose parameter
- removes the concept of a default convention, since in practice both are
  equally found, and requires +convention as soon as a rotational term parameter
  is present.
  For translation only, convention is ignored and optional, as having no effect.
- fixes all the identified uses of proj=helmert in code, doc and tests

This is obviously a breaking change:
- users will have to adapt their pipeline expressions
- in particular, init files that would use helmert must be adapted

However, as designed, the break will be explicit, and not silent.

As mentionned in #1071, it is often unclear how the offset of a vertical grid
is applied.