Merge pull request #1796 from raoulb/partfrac

First version of structured partial fraction decomposition
Merge pull request #1832 from raoulb/expand_harmonic

Expand harmonic
Rewrite of harmonic numbers into summation
Test rewrite of harmonic numbers into sums
XFAIL the test case for this code

The code is correct but due to issues with dummies we can not test that.
Rewrite of harmonic numbers into summation
Test rewrite of harmonic numbers into sums
XFAIL the test case for this code

The code is correct but due to issues with dummies we can not test that.
use replace_dummy in testing an equation with a Dummy
Improve docstring of harmonic numbers and cross refs between numbers in general