FIR filter fit function and description

[FabioAnsaloniQM]

Added the fitting repo for FIR filter developed by Shlomi. Also added an .md file to explain how the fitting works and how to use it. Similar workflow as for the IIR filters

[OziEgri]

@sebastianorbell-qm , you think we can break it to smaller PRs?

[sebastianorbell-qm]

@OziEgri I would recommend splitting into two PR's

1. Documentation (including png's + .md files)
2. Actual code contributions

I think the file structure could be improved to by changing to something like this:

qualang_tools/
│── filters/
│   |── __init__.py
│   |── iir.py
│   |── fir.py
│   |── README.md          # Combined filter documentation
|
|── docs/
│   |── images/
│       |── filters/
│           |── iir_step_response.png
│           |── fir_first_order.png
│           |── fir_second_order.png
│           |── fir_first_correction.png
│           |── fir_second_correction.png
|
── config/
│   |── ...                # configuration utilities

Also removing the .DS_Store files and adding them to the .gitignore

I would be happy to help review the PR's after.

[FabioAnsaloniQM]

@sebastianorbell-qm this sounds good to me. How should we move forward?