LaTeX2e uses 'command' not 'function'

The module really should stick to the LaTeX2e (and presumably
LaTeX3 document-level) naming approach.
Add a first-pass font command approach

This leaves open some questions, particularly in the literal unit part
of the code. Notice that the interface here does allow setting of the
tl for performance reasons.
One 'function' to 'command' missed
Failed to remove a "{#1}"
Merge branch 'master' of github.com:josephwright/siunitx