calibcant/theory.tex: Cleanup after de-appendicizing

calibcant/theory.tex: Cleanup after de-appendicizing

I added a new command \abs{} for absolute values, which uses \lvert
and \rvert.  This works better than |...|, because the lines stretch
to enclose their argument.  The nomencl package also refuses to add
entries containing the pipe symbol to the nomenclature.