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