Add cgs units

These ('Table 9 units') have the same status as e.g. decibel
('Table 8 units'). The BIPM docs imply these are acceptable in
appropriate circumstances, so they should be covered by the module.
Update the 'required LaTeX2e commands' list

The atomic and natural units use a few math mode commands: should
all be OK as these are easy to alter in any case.