Removed dependency on pandas, instead use generic csv

[SkafteNicki]

Before submitting

• [x ] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
• [x ] Did you read the contributor guideline?
• [x ] Did you make sure to update the docs?
• [x ] Did you write any new necessary tests?

What does this PR do?

Fixes #693
Completely new contributor to this awesome library. Took a stab at this, removing the dependency on pandas since it was only used in 4 files:

• Includes pandas but does not use it
• tensorboard.py. Uses pandas for saving csv files. Change to generic csv library
• lightning.py. Uses pandas for loading csv files. Change to generic csv library
• memory.py. Uses pandas for 'pretty printing' of model summary. Wrote own formatter that somewhat mimics the output of pandas. Below is the output from running the basic model from the pl_examples folder with the old and new formatting:
  
  

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

YES!

[Borda]

nice work, Thx, just some adjustments needed + rebase

[Borda]

LGTM, just waiting for the tests pass...

[Borda]

seems like an issue with CSV paths, @SkafteNicki can you run the tests locally, pytest

[SkafteNicki]

By accident I had hard coded a path variable, which worked for me locally. It should be fixed now.

[williamFalcon]

@SkafteNicki amazing!