Reorganize definitions.units

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.

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.