[WIP] Better handling for missing in colors

[asinghvi17]

Description

This PR aims to support users passing Array{Union{Missing, <: Number}} to a plot's color argument in all circumstances. The usecase is passing vectors from DataFrames straight to color - in many cases, this fails because the vectors have missing values, which then have to be changed manually (which is a chore!)

Changes are:

• Allow numbers_to_colors to pass missings through, and use the nan color to represent them.
• Call replace(color_array, missing => NaN) when presented with such an array in assemble_colors
• Let all CairoMakie functions use calculated_color instead of relying on color.
• Add unit tests for this kind of data

Type of change

Delete options that do not apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• Added an entry in CHANGELOG.md (for new features and breaking changes)
• Added or changed relevant sections in the documentation
• Added unit tests for new algorithms, conversion methods, etc.
• Added reference image tests for new plotting functions, recipes, visual options, etc.