add a new and improved unit conversion mechanism

The "old" method for converting units was to store numerical values
alongside their units in a "Measure" struct which was then passed
through the various layers of the code, from parsing to the graphics
backend. The problem with that approach was that we now have a big
mess where unit conversions are scattered throughout the whole
codebase and it's often unclear at which point a given value should
be converted.

The new approach will be to convert all user-specified values into
internal (unitless) numbers as soon as possible; ideally right
after parsing. The conversion from all user-specifiable units to
the internal unit system is clearly defined using the UnitConvMap
struct. During export, all values should then be converted back from
the internal unitless system to the requested output coordinate
system and units.

This change only adds the first parts of the new unit conversion
system. The refactoring of all existing commands to use the new
mechanism is yet to be done.