improve mineral database documentation

By not displaying all the inherited member functions, the mineral
database help page becomes useful. This commit contains some cleanup in
the .py files. Most notably removing redundant comments mentioning the
source for the minerals.
fix composite sum of fractions

Several things in the check if the fractions as positive and sum to 1.0
were broken:

- the normalization was never applied
- if the sum was smaller than one, no warning was produced
- 1.0-0.8-0.2 produces a negative number, but should be legal
add tests for Composite sum(fractions)!=1.0
Merge pull request #30 from tjhei/mineral_overview

improve mineral database documentation
Merge pull request #32 from tjhei/fix_sum_to_one

Fix Composite fractions not summing to one
Add the ability to specify a different reference temperature
change mgd to include new reference temperatuer option.
Moved one line after doc string.