Constant coeff general linear PDE's- first commit
Added references
Rebased and corrected test failure
Implementing TODO 1 and 4- first pass
Hopefully done with all TODO's
Merge pull request #2109 from Manoj-Kumar-S/pde_general

Solving first order linear general PDE's with constant coefficients
Drop str() printers for factorial/subfactorial/etc (issue 3781)
Fix tests for factorial printing (issue 3781)
Fix docstring tests for factorial printing (issue 3781)
Change signs in test for issue 2934