Standardize measurements and units across the library

[v0dro]

Since rubyplot is backend agnostic, there should be a consistent way of specifying various sizes things in a backend agnostic way that is applicable throughout the library. For example, the size conventions of various markers or the spacing between elements.

The conventions should be followed such that they uniformly apply to both GR and Magick and can be easily understood by end users without much head scratching.