Reimplement macros in terms of commath
Update intro docs to explain usage of other packages
Convert some macros to be implemented in terms of mandi
Implement everything possible in terms of mandi

I needed to work around the fact that mandi doesn't implement some of its
macros when it detects that physymb has been loaded. In this case I just
used \providecommand with definitions copied from the implementation of
mandi.
Implement particle macros in terms of hepparticles
Update documentation to mention implementing in terms of mandi macros
Simplify compatibility features with mandi

This commit changes the implementation for macros which are implemented in terms
of the mandi package, in anticipation of mandi removing its own compatibility
features in the next release.