changed cse so it works for Piecewise also
Fixes double-counting bug in constant_renumber

* dsolve((x+1)*f(x).diff(x) - f(x)- 1,f(x)) now returns correctly
* tests in test_constantsimp and test_ode updated
* doctest corrected
* minor readability editing
nested constantsimp to allow for nonlocal state changes
debatable simple fixes

3 failures in test_ode, 2 in test_constantsimp
All are debatable simplifications.
Custom version of ode.constantsimp

- fixes bugs I had
- DOES NOT PASS TESTS!
Ported old constant collison avoider code for ode

- not tested
improve myconstantsimp of exp(Add())'s
catch cse errors
now handles SOME linear combinations
enforce constant memorization for dsolve

- this prevents some rewriting errors
- also, disable constant renumbering, which
	can not yet use memorized constants.
* 10 tests fail, 2-3 raise errors
classify_pde doc fix
tweak __remove_linear_redundancies style

- also, doc-tweaks
reconfigure test_constantsimp.py tests
enable some constant simplifications in Eq()
doc bug reversion
neutral tweaks so test_constantsimp passes
implement new_constants generator for dsolve

- replaces numbered_symbols
- avoids symbol collisions
re-enable constant renumbering for dsolve
__get_constant_subexpressions sees constant Integrals
fix bug in test for linearity