NJOY class, modifying and adding/segmenting methods to generate NJOY inputs

For example,

`pendf()` is now separated into `pendf_input()` and `pendf_run()`.  Ultimately, pendf_run could be optional (I'm still trying to match what is generated by the original PyNjoy.py to help in this QA).  Will need to setup test_njoy.py to check on input generation.

At this point pendf_input pushes to a `file_data` file.  I'll try to have that specified by the user in the function argument.

Also, minor PEP8 fixes on the file.