redefined bootgen class as bruvomat.
new internal function bruvos_distance
simplified bruvo.dist and bruvo.boot

By piecing off the preparation of data into an S4 object and the
calculation of Bruvo’s distance into a function, much of the bruvo.dist
function could be replaced with three lines.

The new form of the S4 object, originally for bootstrapping purposes
runs faster than the original implementation.
Added phylo.bruvo.dist to depreciated list
Added improvement note.