Markers module: PEP8 fixes and MEP10 documentation fixes

Here is a patch on PEP8 changes and MEP10 compliance of the markers module.

The markers module did not have it's own documentation page for now, as the plots using markers's style imported the docstrings and concatenated them. As we decided not to do this anymore, the module now needs it's own documentation page. This patch implements this.

Cheers,
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