[BREAKING] Hermert: add +convention=position_vector/coordinate_frame, forbids +transpose (fixes #1091)

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.

1 files changed, 1 insertions, 1 deletions

diff --git a/docs/source/resource_files.rst b/docs/source/resource_files.rst
index 10cbcb7b..c0321b18 100644
--- a/docs/source/resource_files.rst
+++ b/docs/source/resource_files.rst
@@ -412,7 +412,7 @@ which then expands to
 
     +proj=helmert +x=-0.0001 +y=0.0008 +z=0.0058 +s=-0.0004
     +dx=0.0002 +dy=-0.0001 +dz=0.0018 +ds=-0.000008
-    +t_epoch=2000.0 +transpose
+    +t_epoch=2000.0 +convention=position_vector
     +t_obs=2010.5
 
 Below is a list of the init files that are packaged with PROJ.