[dist] Update method for concatenating distributions for multiple inputs

[BiancaMorandi]

As reported in comments for #368, a workaround was made in the dist function in order to make it work with pandas >= 0.24.0, < 0.25.0. This version of pandas doesn't allow to concatenate two pandas.Series with a CategoricalIndex if having a different index length and therefore the following code was written to handle this.

Describe the solution you'd like
This workaround will need to be replaced with distributions = pd.concat([distributions, distribution], axis=1) when a breaking change release will be made of the brightwind library and the requirements will be updated to work only with pandas >= 0.25.0.