Added links to missing docs in RST file for the diffgeom module
Better docs for Subs.

- Added examples to resolve confusion regarding the result of subs and the limiting value of the expression at that point
- Added examples to clearly show the difference between substitution and numerical evaluation.
- Added `evalf` in the `See Also` of `Subs` documentation section.
minor doc fix
Merge pull request #2688 from Upabjojr/diffgeom_doc

Added links to missing docs in RST file for the diffgeom module
edit fancysets docstrings
Merge pull request #2649 from hargup/fix_subs_doc

Better docs for Subs.
Merge pull request #2674 from smichr/fancy

edit fancysets docstrings
manualintegrate can now handle Heaviside(x)

- Integral(Heaviside(x),x) == Max(0,x)
- enable simple substitutions
- add manualintegrate tests for Heaviside
- add to docs for improving manualintegrate

Also, rename "Log" integration commands to Reciprical for
	readability in manualintegrate module