Add cov_type and kwargs to BLP object

[SvenKlaassen]

Description

The DoubleMLBLP.fit() object now allows to set kwargs for the OLS.fit() method from statsmodels.
Further the default covariance type is set to cov_type='HC0'.

All cate and gate method pass kwargs to DoubleMLBLP.fit() (and use the same default).

Reference to Issues or PRs

Implements #270

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• The title of the pull request summarizes the changes made.
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[JanTeichertKluge]

Approved. The changes look good and all checklist items are completed. Thanks!