test_plot.py: Cleanup temporary files after test completion
4023: better handling of exponents in nfloat

When nfloat encountered RootOf instances the method
it used to leave the exponents alone didn't work because
RootOf instances don't allow non-univariate expression.
So RootOf(x**dummy_1 + x**dummy_2, 0) fails. The handling
of exponents is now handled in a more straightforward
manner and a test expression was added to solve's suite.
handle RootOf on their own in nfloat
Use _argset in preorder_traversal

This makes atoms much faster on LatticeOps (like And and Or), because it
avoids ever computing the sorted .args, which are internally stored as a
frozenset.  This is implemented as the default, because the docstring says
that if keys=False then no order is guaranteed, but if this becomes an issue,
we should perhaps make keys=True the default and set it to False in atoms (the
order never matters in atoms because it returns a set).
Merge pull request #2542 from asmeurer/traversal_argset

Use _argset in preorder_traversal
Don't print a space after ¬ in the pprint of Not
4046: raise error if abs is not removed in solve

A better alternative is to have a method to rewrite Abs
as Piecewise which could then be solved. (see issue 4046)
Merge pull request #2540 from smichr/4046

solve with Abs and checksol issues
Merge pull request #2549 from asmeurer/Not_pprint

Don't print a space after ¬ in the pprint of Not
Merge pull request #2539 from smichr/4023

4023: better handling of exponents in nfloat
Merge pull request #2517 from amitsaha/cleanup_temp

test_plot.py: Cleanup temporary files after test completion
Add Amit Saha to AUTHORS/aboutus. Welcome to SymPy!
Merge pull request #2550 from smichr/auth

Add Amit Saha to AUTHORS/aboutus. Welcome to SymPy!
Merge branch 'master' into logx-for-lseries
Add logx options after new merges
Explicitly call lseries, not series with n=None