This SCV selector is a generalisation of the univariate SCV
selector of Jones, Marron & Park (1991). For d = 1, 2, 3, 4 and binned=TRUE,
the density estimate is computed over a binning grid defined
by bgridsize. Otherwise it's computed exactly.
For details on the pre-transformations in pre, see
pre.sphere and pre.scale.
If Hstart is not given then it defaults to
k*var(x) where k = $\left[\frac{4}{n(d+2)}\right]^{2/(d+4)}$, n = sample size, d = dimension of data.