Add uniform_inside_sphere_distribution

[Ed Smith-Rowland <3dw4rd at verizon dot net>]

This distribution has come in handy for me.

It relies on uniform_on_sphere_distribution and like it, min and max 
have no real meaning.
Unlike uniform_on_sphere_distribution which really is a random 
multidimensional unit vector users often want to pick the radius of the 
distribution.
Unit radius is a good default and is provided but the user can specify 
radius.

Like the uniform_on_sphere_distribution which is used inside, the 
2-dimensional case uses rejection and higher dimensions use a transform.

Built and tested clean on x86_64-linux.

OK?

Ed

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