Format or generate a distance matrix.
numeric vector or matrix.
x is a vector as created by the
dist function, it converts
it into a fulll square, symmetric matrix.
x is a distance matrix, i.e. square, symmetric amd with zero
diagonal elements, it returns the flattened lower triangular submatrix.
Returns a matrix if
x is a vector,
and a vextor if
x is a matrix.
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