Mixed-phase moments

[tluettm]

#1513

The MeanRadius product produces NaN values if ice (negative volume values) are included in the size range and present in the attributes.

Also the WaterMixingRatio product provides a negative mass for integration over ice and is incorrect for summation of ice and liquid water.

I attached a test for these products.

[slayoo]

Thank you, @tluettm

[slayoo]

@tluettm, please confirm if #1513 solves this?

[tluettm]

I found some issues:

If you add 'volume' as an attribute it leads to this error: " ValueError: attribute 'water mass' is a dummy/derived one, but values were provided"

IWC, n_i and r_i are all 0 even if any signed water mass is less than 0. I modified the test. See here:

main...tluettm:PySDM:mixed_moments_new_test

[slayoo]

thank you!