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

added usage example

bug in example - changed +t_in=gpsweek to +t_in=gps_week

Fixes for webmerc projection (fixes #1078)

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.

This is intended to supersed https://github.com/OSGeo/proj.4/pull/1080
with a number of differences.

What is kept from #1080 is not forcing the ellipsoid_params to be the one
of a sphere. This is not required for correct coordinate computation and
avoid lying on the various distorsion parameters.

For better interoperability with EPSG, we also no longer force the
lam0 parameter to 0, because
https://www.epsg-registry.org/export.htm?gml=urn:ogc:def:method:EPSG::1024
has a provision for it, even if in practice they will always be zero
phi0 should always be zero and is not used by the formulas.

Another difference with the #1080 approach is that we do not force the
WGS84 ellipsoid. Perhaps someone will use webmerc for another planet,
even if that is a crazy idea...