minor doc fix for pde.py
Have piecewise sanitize boolean arguements
Test cse on sanitized Piecewise
Add test for cse ignoring order symbols
cse now ignores some big-O notation
classify_ode gets better test for linear eqs
Fix constant simplifying in dsolve

+ Fixes constant-collision errors in dsolve for good
+ Constant numbering starts with C1, unless specified otherwise
+ Specific checks for common subexpression and linear simplifications
call new constant simplifcation from odesimp
all solvers now use new_constants
fix docstring tests in ode.py
fix constantsimp tests
fix test_ode for new constants
minor test_pde fix