manualintegrate can now handle Heaviside(x)

- [x] use x*Heaviside(x) instead of Max(x,0)

- 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