Modernize ParameterBlock::Plus with std::clamp

Use std::clamp for projecting the updated parameter state onto its
box constraints. This provides a more idiomatic and concise
implementation than separate std::min/std::max loops, especially
when both lower and upper bounds are present.

Change-Id: I1f6248ed8f0313338cf9a1c98d79a023ae281e6c

internal/ceres/parameter_block.h

diff --git a/internal/ceres/parameter_block.h b/internal/ceres/parameter_block.h
index 65bb708..a914087 100644
--- a/internal/ceres/parameter_block.h
+++ b/internal/ceres/parameter_block.h
@@ -236,13 +236,16 @@
     }
 
     // Project onto the box constraints.
-    if (lower_bounds_.get() != nullptr) {
+    if (lower_bounds_ && upper_bounds_) {
+      for (int i = 0; i < size_; ++i) {
+        x_plus_delta[i] =
+            std::clamp(x_plus_delta[i], lower_bounds_[i], upper_bounds_[i]);
+      }
+    } else if (lower_bounds_) {
       for (int i = 0; i < size_; ++i) {
         x_plus_delta[i] = std::max(x_plus_delta[i], lower_bounds_[i]);
       }
-    }
-
-    if (upper_bounds_.get() != nullptr) {
+    } else if (upper_bounds_) {
       for (int i = 0; i < size_; ++i) {
         x_plus_delta[i] = std::min(x_plus_delta[i], upper_bounds_[i]);
       }