`BeamLattice.line` Single Element Py Syntax

[EZoni]

We might need a "wrapping" LatticeElements class that can load a list of elements (instead of only loading [0] and assuming it is of kind BeamLine).

Originally posted by @ax3l in #16 (comment)

[ax3l]

In essence, the question is if we want to allow that in the Python API, BeamLine.line accepts also single elements:

    element1 = pals.Marker(name="element1")
    line = pals.BeamLine(name="line", line=element1)

and not force a list of one:

    element1 = pals.Marker(name="element1")
    line = pals.BeamLine(name="line", line=[element1])

I think in general, forcing a list is fine, unless it is very little work to implement casting a single element to a list on assignment.

Internally, and when serializing to files, it MUST stay a list (per PALS), and that is sensible.