BUG: issues with hash-function for Float64HashTable (GH21866)
The following issues

   1)  hash(0.0) != hash(-0.0)
   2)  hash(x) != hash(y) for different x,y which are nans

are solved by setting:

   1) hash(-0.0):=hash(0.0)
   2) hash(x):=hash(np.nan) for every x which is nan

- [x] closes #21866
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
