Correct nth-order linear ode constants

- use a complex-conjugate accounting to avoid adding extra constants
- test nth-order constants without simplification
- remove XFAIL, now that we fix #6247
- remove XFAIL, now that we also fix #7081
Fix for Issue 7140: plot.append
modify plot.append docstring

Also, make extend more precise in terms of what it accepts.
Improve XFAIL's for test_ode

- add XFAIL test for atan2 in im(_)'s
- add XFAIL bad treatment of exp() by collect.
- fix XFAIL in test_ode related to constantsimp
Re-enable term sorting in odesimp()
cse_main: Check for used symbols, workaround for #5331
Add test
Fix cse with iterator

failing test_multiple_expressions
symbols are lookuped up for reduced_exprs.
Cleanup whitespace
replace Wilds in odesimp() with powsimp()
Simplified test_name_conflict
Add a function to filter out excluded symbols and test.
Use filter_symbols in cse and add test.
Merge pull request #7452 from smichr/7140

fix plot.append
Merge pull request #7447 from zanzibar7/issue_6247

Fix #6247  Too many constants for nth-order linear ode
Merge pull request #7458 from lennart0901/workaround_5331

cse_main: Check for used symbols, workaround for #5331
Add Lennart Fricke to AUTHORS/aboutus. Welcome to SymPy!