Add an inverted dictionary and a lookup function from awg -> mm2. Also
do some minor refactoring. Both sides of the conversion table were
converted to strings, since '0000' and '2/0' are perfectly valid AWG
values.

Show conversions for ex02, and make sure it displays conversions in both
directions. Rebuild the example files.

The parsing allows arbitrary units to be used for cable dimensions --
this might be valid units, e.g. square inches, or invalid, e.g. bananas.
We only allow conversion between mm2 and AWG, so check that the
gauge_unit is either of those before conversion. If not, pass through as
is.

Convert e.g. 'awg, 'AwG' to upper case for consistent rendering. Leave
any other input gauge units as they were.