Restructure API and support backward compatibility

[Wenzhi-Ding]

Hi Alex, as we discussed in #329, here is the pull request for restructuring API. It also supports existing API used in your Jupyter Notebook, with a FutureWarning. Here are a list of major changes:

1. Most changes are done in __init__py for the package and each sub-modules.
2. I rename some files under estimation module to avoid namespace conflict. For example, detect_singleton.py contains function detect_singleton(). Organic calling it will be pyfixest.estimation.detect_singleton.detect_singleton. To avoid such ambiguity between sub-module name and function name, I rename detect_singleton.py to detect_singleton_.py
3. I re-run the codes in readme and Jupyter Notebook to ensure both old and new API works.
4. I correct a typo from "pplmhdfe" to "ppmlhdfe".

There are lots of files changed. The review may take lots of time. Thank you so much for considering this PR!

Best regards,
Dave

[codecov]

Codecov Report

Attention: Patch coverage is 88.52097% with 52 lines in your changes are missing coverage. Please review.

Project coverage is 86.19%. Comparing base (35b47e9) to head (b8394f2).
Report is 19 commits behind head on master.

Files	Patch %	Lines
pyfixest/utils/_exceptions.py	0.00%	15 Missing ⚠️
pyfixest/report/summarize.py	95.13%	9 Missing ⚠️
pyfixest/multcomp.py	0.00%	8 Missing ⚠️
pyfixest/summarize.py	0.00%	8 Missing ⚠️
pyfixest/visualize.py	0.00%	7 Missing ⚠️
pyfixest/report/visualize.py	96.22%	2 Missing ⚠️
pyfixest/did/event_study.py	0.00%	1 Missing ⚠️
pyfixest/estimation/feols_.py	85.71%	1 Missing ⚠️
pyfixest/estimation/multcomp.py	98.43%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #332      +/-   ##
==========================================
+ Coverage   85.79%   86.19%   +0.40%     
==========================================
  Files          38       53      +15     
  Lines        3421     4043     +622     
==========================================
+ Hits         2935     3485     +550     
- Misses        486      558      +72     

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[s3alfisc]

Thanks for spotting this typo. I was convinced it was pp l m hdfe 😄

[s3alfisc]

Not sure what is going on with the tests on 3.10. I will merge now nevertheless as the PR looks great. As I said before, in my experience, these errors arise occasionally and tend to go away by themselves. Though we should make sure to fix this before the next Pypi release =) Thanks for the PR Dave!

[Wenzhi-Ding]

Thanks for merging the PR! This code actually passes Python 3.10 test several hours ago, but after I made a small correction in typo, the test breaks again...Quite confusing. Hopefully we can figure out what's going on...I will take a look at this problem in the following few days.