plot functions now pass on kwargs when constructing series.
Added `to_matrix` method to Vector and Dyadic in mechanics.

When jumping back and forth from symbolics to numerics when using the mechanics
module it would be nice to be able to output the matrix forms of vectors and
dyadics with respect to different reference frames. I've added two new methods
that provide this functionality. It is particularly useful for returning
inertia tensors in reference frames from dyadics as the tensor form in the more
widely used.
Changed syntax on the doc test so it now passes.
gh2666: terms_gcd does not remove and _keep_coeff doesn't hold 1.0

In addition, if only a 1 (or 1.0) is extracted, the original
expression is returned from terms_gcd rather than the polys-modified
form (e.g. 2.0*x + 3 instead of 2.0*x + 3.0).
Solvers: removed duplicate output

Fixes issue 4129 by passing result through set before returning
Corrected position of ordered and set tomaintain canonical ordered list
Simplified the code with list comprehensions.
Added notes in documentation about .
Added test cases for issue 4129
Removed redundant test cases
fix generate_bell with recursive algorithm

TODO: replace with a non-recursive algorithm.
Document mathematical constants singletons
Standard reference for NaN
Reword some docstrings, mention that they are singletonized.
Add missing numbers singletons to sphinx docs
Add examples for numeric singletons
Merge pull request #2664 from skirpichev/singleton-docs-numbers

Document mathematical constants singletons
Merge pull request #2686 from moorepants/dyadic-2-tensor

Added `to_matrix` method to Vector and Dyadic in mechanics.
keep 1.0 and -1.0 with _keep_coeff
Merge pull request #2689 from smichr/gh2666

gh2666: terms_gcd does not remove 1.0