Add functions for calculating angle between two vectors

[frutiemax]

It would be nice to have member functions of the Vector classes to calculate the angle between two vectors.

[Perksey]

cc @dfkeenan given that you're proposing some mathsy things!

[aquagoose]

This is pretty much what the dot product does.

[Perksey]

Oh. In that case this may not be needed, but I'll leave this open because it's possible that it can be paired with #1169 for explicitness (unless @dfkeenan comments otherwise)