Add `equal_var` for hypothesis tests

[tompollard]

In #189, @vljlangen suggests adding an equal_var for hypothesis tests.

In addition, if it's at all feasible, having control over equal_var (with False as the default) would be really useful as well. Maybe even with a small warning or note like:

"Note: the default behavior of ttest_ind in Python is equal_var=True, which might explain differences in p-values between tableone and manually run tests."

That could help avoid confusion for users who are comparing results across different implementations.

[tompollard]

Added in #194.

[tompollard]

@vljlangen Looking at the code, the test previously defaulted to equal_var=False (without the option to set it to True).

tableone/tableone/statistics.py

Lines 166 to 171 in ccdb5ca

	if (is_continuous and is_normal and len(grouped_data) == 2
	and min_observed >= 2):
	ptest = 'Two Sample T-test'
	test_stat, pval = stats.ttest_ind(*grouped_data.values(),
	equal_var=False,
	nan_policy="omit")

There is now an argument that allows equal_var to be set to True (but default is still False).

[vljlangen]

That’s fantastic, thank you so much for adding that! It’s really nice to have that control now—it makes it easier to line things up with results from elsewhere. I really appreciate the quick update and thoughtful support!