SpinMC duplicating

[JeffFessler]

It is OK that SpinMC expects τ but stores r internally, but this makes it hard to make a copy of one of those and change something other than r.

There could be an inner constructor that takes r directly, and an outer convenience constructor that lets the user use τ.

Or there should be a method that recreates the original τ from spin.r, to use to construct a new SpinMC.