Add boostCM_of to clarify #134, supported by scale*D and neg*D.

[jpivarski]

Adds the following to 2D vectors:

• scale2D: same as scale
• neg2D: same as scaling/multiplying by -1

Adds the following to 3D vectors:

• scale2D: only scales the azimuthal components
• scale3D: same as scale
• neg2D: only negates the azimuthal components
• neg3D: same as scaling/multiplying by -1

Adds the following to 4D vectors:

• scale2D: only scales the azimuthal components
• scale3D: only scales the azimuthal and longitudinal components
• scale4D: same as scale
• neg2D: only negates the azimuthal components
• neg3D: only negates the azimuthal and longitudinal components
• neg4D: same as scaling/multiplying by -1
• boostCM_of_p4(p4): boosts to the center-of-mass (CM) frame of p4 (same as boost_p4(p4.neg3D))
• boostCM_of_beta3(beta3): boosts to the center-of-mass (CM) frame of beta3 (same as boost_beta3(-beta3))
• boostCM_of(booster): uses one of the two above, depending on the dimension of the booster

The main motivation was for a method that boosts to the center-of-mass frame of a given vector, as that was difficult to do with boost (you had to negate the spatial components only, and there wasn't a method/property for that). The rest of this is to support that function, and for consistency.

All of the associated Numba methods have been implemented as well. (None of the above involved any new compute kernels; it was just applying the right ones to each type.)