Add warning when predict has a type mismatch

[ivanhigueram]

As discussed in (#731), when predict's newdata has a different dtype in the FEs, the function will yield missing values only, this because predict relies on a dictionary of FEs where keys are transformed to strings and will fail to get the correct value (i.e. 10 vs "10.0").

This commit adds a warning in the predict method and a simple unit test that checks for this case for the user.

[s3alfisc]

This looks great, thank you @ivanhigueram!

@all-contributors please add @ivanhigueram for code =)

[s3alfisc]

pre-commit.ci autofix

[s3alfisc]

We get new mypy errors - I just fixed those in my local branch of #758 =D Will push a patch!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Flag	Coverage Δ
core-tests	78.12% <100.00%> (+0.02%)	⬆️

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Files with missing lines	Coverage Δ
pyfixest/estimation/feols_.py	83.72% <100.00%> (+0.08%)	⬆️
pyfixest/estimation/model_matrix_fixest_.py	85.08% <100.00%> (ø)

[s3alfisc]

pre-commit.ci autofix

[s3alfisc]

@all-contributors please add @ivanhigueram for code

[s3alfisc]

Oh wow changes are clearly not working, I'll tackle this tomorrow