[ENH] Make filters available in Orange.data namespace.

[thocevar]

Issue

It would be convenient to access filters in the same way as other classes from the data module.

import Orange
tab = Orange.data.Table("iris")
filter = Orange.data.SameValue("petal length", 5.1)
print(len(filter(tab)))

Description of changes

Import everything from filters in data/__init__.py.

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Codecov Report

Merging #3170 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3170      +/-   ##
==========================================
+ Coverage   82.54%   82.54%   +<.01%     
==========================================
  Files         337      337              
  Lines       58431    58432       +1     
==========================================
+ Hits        48229    48230       +1     
  Misses      10202    10202

[lanzagar]

If you want to import * from filter, you have to add an __all__ declaration there.