Gridlines support linestyle

[daniel-stoneuk]

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Added support for dashed (and other) types of gridline. Fixes #498.

Changes

• Updated pre-commit config
• Changed unsupported use of minor linestyle in logplot test
• Moved cmap conversion code to _color.py to avoid circular import
• Support dashed (and other) types of gridline
• Added tests

Notes

I was unsure where to put the gridline_options method so please feel free to move this elsewhere if desired.