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 f90e94de2ff8c7e3fa2ed8cf3243bff62545cde4 on ghk829:master into f6f92d763ab385013b72776acf990857d4949e66 on rhiever:master.