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.

Parameters for the plate motion models (PMM) for ITRF2008 and ITRF2014
are added to the ITRF2008/2014 init-files. The PMMs allow coordinates
to be moved back and forward in time in plate fixed reference frames
such as GR96 in Greenland which is defined as ITRF94@1996.623.
Transforming an ITRF2014-coordinate to GR96 is done with:

+proj=pipeline +step +init=ITRF2014:NOAM   +t_epoch=1996.623 +t_obs=2017.584
               +step +init=ITRF2014:ITRF94 +t_obs=1996.623

where the first step transforms the coordinate back in time to
ITRF2014@1996.632 by using the ITRF2014 PMM. The second step transforms
the ITRF2014 coordinate to ITRF94.

Prefixing with t_ is more consistent with the existing parameters
in PROJ, such as x_0 and friends. t_epoch and t_obs is already used
in PJ_deformation. Now users can expect consistency across
time-varying transformations.

* Add ITRF init files, that makes it easy to use ITRF transformations in pipelines

* Added ITRF* files to the build systems