Consider alternative parameter names for the Helmert operator

[busstoptaktik]

The Helmert operator can take up to 16 parameters in the dynamic case. This makes expressions involving Helmert steps quite hard to read. And as the parameters typically come in families of 3, we might as well introduce an optional set of parameter names, each taking a group of 3 numbers.

For instance, we may replace the group

x=0.03054 y=0.04606 z=-0.07944
rx=0.00141958 ry=0.00015132 rz=0.00150337
s=0.003002

by the more easily eaten

translation = 0.03054, 0.04606, -0.07944
rotation = 0.00141958, 0.00015132, 0.00150337
scale = 0.003002

perhaps adding homologous names like velocity and angular_velocity for the dx, dy, dz and drx, dry, drz pairs.

Or what about translation_trend and rotation_trend?

For PROJ compatibility, the existing parameters should be kept as alternatives.

Tasks

• [x] Implement functionality (@nrhill1 in #91)
• [x] Update tests (@nrhill1 in #91)
• [x] Update documentation