add plot_layout basic functionality

[cnrrobertson]

Addresses #61. This is an implementation of the patchwork plot_layout function that allows for controlling the number of rows, columns, in which direction plots are inputted (row-wise or column-wise), and their relative widths and heights. It is still missing some pieces:

• Ability to leave an empty height or width and it be auto filled
• Guides, tags, axes, axis_titles, and design functionalities from patchwork
• Docs of course!

I'm not sure how much of the above is currently feasible with Makie's SpecApi tools (like manipulating shared axes between plots in a GridLayout), so it would probably have to be done as post processing somehow. Or maybe time for some upstream contribution!

In the meantime, this does work with both GGPlot and GGPlotGrid inputs but it's most straightforward to use only + operations.

[rdboyes]

Hey @cnrrobertson - I'm inclined to merge this, close the initial issue that called for "basic patchwork functionality" and add issues to track your future changes. Is there anything stopping this from being included as it is? Its ok if its a "work in progress" - the whole package is, after all

[cnrrobertson]

Sorry for the delay @rdboyes, I have been traveling. It is functional as is, so I see no reason it couldn't be merged. I can put the checklist above as a new issue.

[rdboyes]

No need to apologize! I'm merging this now - thanks for the contribution