reliability model: distinguish decimal from binary units (e.g. KB/KiB)

Signed-off-by: markkampe <mark.kampe@inktank.com>
reliability models:	first pass at breaking out failure probabilities
	durability method now returns a vector of tributary probabilities
	durability method now returns a vector of tributary losses
	all now have useful and raw size to permit loss per petabyte

Signed-off-by: markkampe <mark.kampe@inktank.com>