Merge pull request #1 from rth/master

added a couple of fuctions to opacplot2
Added some more physical constants including:

- Electron mass (opp.ME)
- Planck's constant (opp.PLANCK)
- hbar (opp.HBAR)
Extended the IONMIX format to support specific electron entropy.
Added explain which demonstrates how to generate data for an IONMIX
file which includes electron entropy information.
Fixed bugs in two example problems