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
Shift two issue numbers in strings like issue([0-9]+), see faedd55

I'm not sure about the #4021 (old issue number 922), maybe
it's #4087.  This issue number looks as a typo for me (998 instead
of 988 in the old bugtracker).  Test was added in the #4021 (commit
a945bc6), but it works only with modifications from #4087 (issue
988, commit 3741e80).

[skip ci]