Reorganize definitions.units

[tiffany352]

Changes to definitions.units

• Added vim folds repurposing the !category/!endcategory syntax. There's a modeline at the bottom of the file for this.
• To make the vim folds work better, every single unit now has a category. I also moved them around a little bit so that each category collapses to a single line. This makes the file much easier to navigate. Kind of acts as its own table of contents.
• Rearranged some of the categories, and moved things between categories.
• Adjusted several comments, and added an explanation of the file to the top.
• Deleted some legacy GNU Units directives like !utf8 that Rink will never use. Also several of the function/LUTs that are commented out.
• Uncommented some fractions that apparently GNU Units didn't like. Also uncommented googol.
• Moved mercury to the periodic table section.
• Moved the 4 newest elements to be alphabetically sorted.
• Added atomic numbers to every element. (I double checked that the numbers are correct, but I might have still gotten some mixed up...)
• Fixed a few typos and trailing whitespace.

Changes to rink-core

• Now prints an error on unrecognized directives instead of silently ignoring them.
• Ignores categories for non-units. This may be re-added later but it will require refactoring Context::categories to respect namespaces.
• Improved some of the error messages from the units DB loader.