BUG #15150  normalization of crosstable with multiindex and margins
 - [x] closes #15150
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [x] whatsnew entry

When debugging this issue I came across some unexpected results for margins 
when normalization 'index' or 'column' is performed. Here a cross table with 'column' normalization (example from line 1271 in test_pivot.py):

    b    3    4  All
    a               
    1  0.5  0.0  0.2
    2  0.5  1.0  0.8

I would expect, that margin values should always be the sums of rows/cols, regardless if values were normalized or not, so I would expect the following:

    b    3    4  All
    a               
    1  0.5  0.0  0.5
    2  0.5  1.0  1.5

This is not the case for 'index' and 'column' normalization. In fact, margin values are calculated as sums of raw values and then normalized. This is fine for normalization 'all'. But for normalization 'columns' and 'index', this leads to -- at least for me -- unexpected results.

I left the calculation as it is, because this kind of behavior is validated in test_crosstab_normalize
(test_pivot.py), so maybe the calculation is wanted like that. Is it?
 