v.perp(direction) should compute 2D magnitude along an alternate 3D longitudinal axis direction (both are at least 3D)

[jpivarski]

From ROOT's TLorentzVector:

>>> import ROOT

>>> v = ROOT.TLorentzVector(1.1, 2.2, 3.3, 4.4)
>>> direction = ROOT.TVector3(3, 2, 1)

>>> v.Perp(direction)
2.880476151115099

We want the same thing in a convenient method: v.perp(direction)

>>> import awkward as ak
>>> import numpy as np
>>> import vector
>>> vector.register_awkward()

>>> v = ak.Array([
...     [{"x": 1.1, "y": 2.2, "z": 3.3}], [], [{"x": 1.1, "y": 2.2, "z": 3.3}]
... ], with_name="Vector3D")

>>> direction = ak.Array([
...     [{"x": 3, "y": 2, "z": 1}], [], [{"x": 3, "y": 2, "z": 1}]
... ], with_name="Vector3D")

>>> v.mag * np.sin(v.deltaangle(direction))
<Array [[2.88], [], [2.88]] type='3 * var * float64'>

New "compute" modules are not needed; just compute this as a convenience method from the existing ones.