[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/src/proj_4D_api.c b/src/proj_4D_api.c
index a6a8dc27..f0543893 100644
--- a/src/proj_4D_api.c
+++ b/src/proj_4D_api.c
@@ -503,7 +503,7 @@ Returns 1 on success, 0 on failure
         def = malloc (100+n);
         if (0==def)
             return 0;
-        sprintf (def, "break_cs2cs_recursion     proj=helmert exact %s transpose", s);
+        sprintf (def, "break_cs2cs_recursion     proj=helmert exact %s convention=position_vector", s);
         Q = proj_create (P->ctx, def);
         pj_inherit_ellipsoid_def (P, Q);
         free (def);