Returns a measure of spread associated with the circular median of a
vector of circular data.
Details
See equation (2.33) at pag. 36 in Fisher (1993)
for its definition. In the case the circular median is not defined, that
is, every angle is a median axis, the mean deviation is not reported and
set to NA.
References
N.I. Fisher (1993) Statistical Analysis of Circular Data, Cambridge
University Press.