Change Minimizer::Options::min_trust_region_radius to double.

This was accidentally an int, which was setting the minimum
trust region radius to zero and effectively disabling a convergence
test based on it.

(Thanks to Sergey Sharybin for providing a reproduction for this)

Change-Id: Id0b9e246bcfee074954a5dc6a3a2342adab56c16
1 file changed