Be able to change lines thickness translation

[LeanderK]

The translation between matplotlib and tikz is, as far as I can see, defined here:

https://github.com/texworld/tikzplotlib/blob/450712b4014799ec5f151f234df84335c90f4b9d/src/tikzplotlib/_path.py#L409

I have different thickness defaults in my tikz-plots, so the translation is out of sync. This forces me to change the thickness of my matplotlib lines, resulting in pretty unreadable matplotlib plots just to get the thickness correct. It would be great to change those things globally! I can create a PR if this is a welcome addition and you would give me a hint on how you would like to have it implemented. I was thinking of something simple like matplotlibs rcparams but I am happy to do something else.