Plot unbroken lines for E and J fields

[santisoler]

Currently some DC widgets plot broken lines for the E and J fields:

This plots could be misleading. Since lines density relates to the magnitude of the fields, having broken lines appearing and disappearing could lead to wrong interpretations of the values of the field in different regions. For example, in this image, the fields lines are less denser around the sources than in deeper regions.

We should pass broken_streamlines=False to matplotlib.pyplot.streamplot when plotting field lines.

This option was introduced in matplotlib 3.6, therefore the pinned version of matplotlib should be changed so we can make this change. Is there any reason why matplotlib is pinned to <3.5 on environment.yml? It was pinned in #45

[micmitch]

Nice that they have added this functionality to streamplot!