Allow covariates in plot_expected_purchases

[PabloRoque]

Description

Includes covariates when calculation the unconditional frequency expectation. Allows the estimation of future purchases for the "average customer" just after first order for models with covariates.

Additionally:

• Introduces a new dataset apparel_trans.csv and accompanying apparel_static_cov.csv. These datasets are extracted from R's CLVTools package.
• A new notebook bg_nbd_covariates.ipynb shows an example of the new functionality using the new dataset.

Related Issue

• Closes #
• Related to Allow static covariates in BGNBDModel #1390. That PR should be merge before this one.

Checklist

• Checked that the pre-commit linting/style checks pass. Feel free to comment pre-commit.ci autofix to auto-fix.
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks) using numpydoc format.
• If you are a pro: each commit corresponds to a relevant logical change

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📚 Documentation preview 📚: https://pymc-marketing--1430.org.readthedocs.build/en/1430/

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Codecov Report

Attention: Patch coverage is 16.12903% with 78 lines in your changes missing coverage. Please review.

Project coverage is 56.03%. Comparing base (cf3eab2) to head (d31d8d3).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pymc_marketing/clv/models/beta_geo.py	21.12%	56 Missing ⚠️
pymc_marketing/clv/utils.py	0.00%	22 Missing ⚠️

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Flag	BASE (cf3eab2)	HEAD (d31d8d3)
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Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1430       +/-   ##
===========================================
- Coverage   92.58%   56.03%   -36.56%     
===========================================
  Files          52       52               
  Lines        6043     6109       +66     
===========================================
- Hits         5595     3423     -2172     
- Misses        448     2686     +2238     

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[ColtAllen]

I'll review this in detail later, but it looks like there may be merge conflicts with #1390