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

internal/ceres/minimizer.h

diff --git a/internal/ceres/minimizer.h b/internal/ceres/minimizer.h
index 708974d..040ddd9 100644
--- a/internal/ceres/minimizer.h
+++ b/internal/ceres/minimizer.h
@@ -113,7 +113,7 @@
     DumpFormatType lsqp_dump_format_type;
     string lsqp_dump_directory;
     int max_num_consecutive_invalid_steps;
-    int min_trust_region_radius;
+    double min_trust_region_radius;
     LineSearchDirectionType line_search_direction_type;
     LineSearchType line_search_type;
     NonlinearConjugateGradientType nonlinear_conjugate_gradient_type;