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.