Solving variable coefficient PDE's
Also made slight changes in documentation
Replaced * with spaces
Used __name__ instead of class_key
Made the following changes
1] Corrected two minor bugs in pdsolve
2] Extended 1st_linear_varcoeff_pde for the following subcases
   a] d = 0, and one of b and c is 0
   b] b = 0
   c] c = 0
   in the PDE b*u.diff(x) + c*u.diff(y) + d = e
Added removed test
Corrected minor bugs in docs
Merge pull request #2346 from Manoj-Kumar-S/varcoeff_pde

Solving variable coefficient PDE's