Multipole Parameter Helpers

[ax3l]

After #44 and #38, we could add a few more helpers to *MultipoleParameters.

Query

Since the defined attributes are dynamic, it is not entirely trivial to iterate over them without doing the string parsing again of the keys.

We should add helpers of up to which order components are defined and getters to read them without parsing the string keys manually (and adding 0-value defaults for orders without definition).

Conversion

Let implementers query the B <-> K and length-integrated / non-integrated values of the electric and magnetic multipole parameters, independent of what was passed by the user.

Implement these relations:

See: https://pals-project.readthedocs.io/en/latest/element-parameters.html#magneticmultipolep-magnetic-multipole-parameters

The user will have to pass reference particle information for the conversion, so we cannot make it as easy as a virtual .property