add simple case for Integral.is_zero
Merge pull request #7440 from smichr/is_zero

add simple case for Integral.is_zero
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 simplification from odesimp
all ode solvers now use new_constants
fix docstring tests in ode.py

- remove broken separable-reduced test
fix constantsimp tests
fix test_ode for new constants
minor test_pde fix
fix arbitary-order linear ode solvers constants

- now, use a list of symbols rather than a generator
refactor get_new_constants procedure
fix constant numbering of ordinary series tests
allow 'exclude' keyword for numbered_constants
terms_gcd no longer converts Eq to Expr when factoring
allow numbered_symbols to exclude symbols
spelling changes
review-related minor modifications