Need coordinate transformation functions with momentum names

[jpivarski]

I just wrote up an example that transformed a Cartesian Lorentz vector into cylindrical coordinate system with pseudorapidity and mass, and I had to say

lorentz_vector.to_rhophietatau()

although we had been talking about "pt", "phi", "eta", and "mass" coordinates earlier. What's worse, the Awkward backend makes these the field names. All 12 conversion functions should have momentum-named equivalents that ensure that the output array has momentum field names.

[matthewfeickert]

This came up in Discussion #324, and while I assume that there is some non-trivial amount of work for this, it would be quite nice to have.

[matthewfeickert]

I don't have time to contribute a PR for .to_ptetaphim in the next weeks, so while I'm hoping that someone else beats me to it(!), I'll also drop some self reference points to come back to, like the way that .to_rhophietatau is implemented is here:

vector/src/vector/_methods.py

Lines 335 to 340 in 1c5bd52

	def to_rhophietatau(self) -> VectorProtocolLorentz:
	r"""
	Converts to $\rho$-$\phi$-$\eta$-$\tau$ coordinates, possibly imputing
	dimensions with a projection.
	"""
	raise AssertionError

vector/src/vector/_methods.py

Lines 2940 to 2955 in 1c5bd52

	def to_rhophietatau(self) -> VectorProtocolLorentz:
	from vector._compute import lorentz, planar, spatial
	lcoord = 0
	if isinstance(self, (Vector3D, Vector4D)):
	lcoord = spatial.eta.dispatch(self)
	tcoord = 0
	if isinstance(self, Vector4D):
	tcoord = lorentz.tau.dispatch(self)
	return self._wrap_result(
	type(self),
	(planar.rho.dispatch(self), planar.phi.dispatch(self), lcoord, tcoord),
	[AzimuthalRhoPhi, LongitudinalEta, TemporalTau],
	1,
	)