add fill_func to fillna

[ghk829]

What does this PR do?

I add the parameters a fill_func for filling NA

default is the same as the original
and
we can expand the function to fill the NAs

Where should the reviewer start?

the constructer of the autoclean function

How should this PR be tested?

I add the test function in tests.py

Any background context you want to provide?

 python dynamically import and numpy nanfunctions modules

What are the relevant issues?

[you can link directly to issues by entering # then the number of the issue, for example, #1 links to issue 3]

Screenshots (if appropriate)

Questions:

• Do the docs need to be updated?
  If my pull request is admitted, I will add docs
• Does this PR add new (Python) dependencies?
  Nope.

[coveralls]

Coverage increased (+1.1%) to 80.377% when pulling f90e94d on ghk829:master into f6f92d7 on rhiever:master.