tex, string, and separator options for units

[Alex-Jordan]

This implements one of the suggestions from @dpvc in #1437, and replaces that pull request.

• Units defined in Units.pm can have string, tex, string_separator, and tex_separator properties.
• All aliases for a unit will carry over these properties. To demonstrate what to do when this is unwanted, I moved micron to be a named unit instead of an alias.
• The string property is used when the unit or its aliases are printed as a string.
• The string property needs to use a string that can actually be printed in hardcopy. There is a comment about this in the file above where the big units hash is defined.
• Since a unit may be interpolated as part of an argument to Compute(), make sure that any all string properties are also unit names.
• The tex property is used when the unit or its aliases are printed as tex.
• The tex_separator property are only used when the unit is not part of a larger fractional unit, and the unit is the first unit following a number. Otherwise a space is used, as has been the case. For example, 180° and 180° s, but 180 s ° (except as tex).
• The string_separator property is similar, except it is also used even when there is a larger fractional unit. For example, 180°/s.
• Only deg is using tex_separator and string_separator, with each set to the empty string. I considered doing this with degF and degC, but Google tells me there should be a space for those units for scientific publications.
• contextUnits.pl was adding aliases for L (liter, liters, litre, and litres). It was cleaner to move those over to Units.pm. If there was a reason they were not already in Units.pm, I could move them back.

One thing that could still be improved, is that for string production, the spaces that are used in between numbers and units (when string_separator is not set), or between units and units should be nonbreaking spaces. I tried things that I though should work, but did not work for one reason or another. So that can be addressed in some future PR.

Try the problem from #1437 for some basic testing. Although you may want to also try with some more complicated units that have unit products and quotients.