Add the ability to specify a different reference temperature
change mgd to include new reference temperatuer option.
Moved one line after doc string.
Merge pull request #33 from ian-r-rose/different_reference_temperature

Different reference temperature

Though the great majority of mineral physics experiments are with 300. K as the reference temperature, some (especially with metals) use others. I've added the ability to specify T_0 in the params object. If it is not specified, we just default to 300. K. This should be completely backwards compatible.