Combinatoric Cheatsheet

A combinatoric cheatsheet. It include methods names and a short
description.
Merge branch 'master' of https://github.com/Mba7eth/sympy into comb_cheat
Updated Makefile for combinatoric cheatsheet.
Fix latex subscript, etc use in ode.py lines 3671-2

Fixes #7417
Made some edits of latex and edited desription of ode_1st_power_series doc

More Fixes for #7417
Whitespace removed in ode.py
Modified func_coef
Added first method for linear system 2nd order 2eqs
Fix too long line.

On line 2782 in ode.py there was a too long line. I separated the line into two
Added first method for linear system 2nd order 2eqs
Added method to solve type2 of 2nd order
Resolved conflict
Modification in test cases and is_linear match
Fixed the subscripting for x_{0}.
Fixed need for :math: in 5386
Added some methods for linear system of 2 eqs for 2nd order
Modified func_coef
Added first method for linear system 2nd order 2eqs
Added method to solve type2 of 2nd order
Modification in test cases and is_linear match