Fix to minor bug in simulated data generation function

[drbenvincent]

Closes #433

P.S.: In the same function, should the input intercept (i.e. beta_intercept) in the output dataframe instead of the constant 1

I believe the answer to this is "no". The beta_intercept input is being used in the data generation process, but the purpose of df["intercept"] = np.ones(df.shape[0]) is to create input data for modelling where it's common to create a column of zeros. Though practically I think this is not needed or is even being used because it's being taken care of by patsy based on the user-provided model formula. But for the moment I've just added an explanatory comment.

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📚 Documentation preview 📚: https://causalpy--505.org.readthedocs.build/en/505/

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
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Files with missing lines	Patch %	Lines
causalpy/data/simulate_data.py	0.00%	1 Missing ⚠️

Additional details and impacted files

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[ErikRingen]

LGTM