VOTE - v4 new names for getBv and getHv

Unfortunately v3 was a bit premature.

With Box -> Cuboid (its a must) the API is broken and we have to move to v4.0.0 with the next patch. After a thorough review of all features (see #360 ) the only remaining namechange must be of getBv and getHv.

These top-level functions avoid the object oriented interface and give direct access to the analytical formulas based on complex dictionary inputs. Its the fastest way to compute the field in Magpylib if you know how to vectorize code properly (but even then not much faster than the object oriented interface).

Please propose names @lucasgcb @pribossek @Alexboiboi

getB_direct ? getB_a_little_faster ?

[OrtnerMichael]

@lucasgcb Well in Magpylib2 we added vectorized code for the first time in the getB_Box_vector() functions. The name getBv() derivates from that as the functionality (dictionary input) is quite similar. However, now everything is vectorized automatically all the time and getBv() is just there for users who want to squeeze out all possible performance by avoiding the object oriented interface overhead. The performance gain is not massive anymore (factor 1.1-2), and you only get that if you properly generate the input using np.tile and similar commands instead of for loops.

[OrtnerMichael]

completed with #375