Floats - neither rational nor irrational

A Float represents an interval of real numbers, both rational
and irrational. It cannot be meaningfully tested for rationality.
Replaced 'interval' with less exact expression.
Add wavenumber property
Add equation rewrite methods
Add test for rewrite expr using cos
Add spatial component in representation
Add rewrite as exp function
Add more tests
Merge pull request #7546 from debugger22/wave-equation

Enhancing TWave
Merge pull request #7331 from jksuom/floats

Floats - neither rational nor irrational
Added doctests
Merge branch 'master' of https://github.com/sympy/sympy into basicvector
Rectified doctests