Remove median(::MvLogNormal)

[devmotion]

The PR removes the definition of median(::MvLogNormal). It was added in #455 but IMO that should have been discussed more thoroughly.

1. The marginal median is just one of multiple concepts of multivariate medians, see e.g. https://en.wikipedia.org/wiki/Median#Multivariate_median.

2. There's no established concept of multivariate medians in Distributions as indicated by the fact that median is not defined for any other multivariate distribution (not even MvNormal).

3. The definition of median seems to suggest that quantile etc. should be defined as well, which is not the case.

As such, I think the existence of this definition is a bug.

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Alternatively, Distributions could adopt and clearly document a definition of (marginal?) medians for multivariate distributions. In this case, the fix would be to add definitions of median to other distributions such as MvNormal etc. as well.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.27%. Comparing base (da3a230) to head (352c972).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1966   +/-   ##
=======================================
  Coverage   86.27%   86.27%           
=======================================
  Files         146      146           
  Lines        8777     8771    -6     
=======================================
- Hits         7572     7567    -5     
+ Misses       1205     1204    -1     

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