[numeric] minMaxMeanMedian speed improvement openMVG#1398

- Add unit test
- Use nth_element and min/max_element to minimize the number of
operation

A faster solution could be to use somthing like the following:
```
T min = a[0], max = a[0];
std::nth_element(a.begin(), a.begin() + n, a.end(),
    [&](T lhs, T rhs) {
        min = std::min(min, std::min(lhs, rhs));
        max = std::max(max, std::max(lhs, rhs));
        return lhs < rhs;
    });
```
but since nth_element is not guaranted to look to all element in the
array I prefer rely on 3 STL operation.

src/openMVG/numeric/numeric.h

@@ -370,16 +370,18 @@ bool minMaxMeanMedian( DataInputIterator begin, DataInputIterator end,
   }
 
   std::vector<Type> vec_val( begin, end );
-  std::sort( vec_val.begin(), vec_val.end() );
-  min = vec_val[0];
-  max = vec_val[vec_val.size() - 1];
-  mean = std::accumulate( vec_val.begin(), vec_val.end(), Type( 0 ) )
+
+  // Get the median value:
+  const auto middle = vec_val.begin() + vec_val.size() / 2;
+  std::nth_element(vec_val.begin(), middle, vec_val.end());
+  median = *middle;
+  min = *std::min_element(vec_val.begin(), middle);
+  max = *std::max_element(middle, vec_val.end());
+  mean = std::accumulate( vec_val.cbegin(), vec_val.cend(), Type( 0 ) )
     / static_cast<Type>( vec_val.size() );
-  median = vec_val[vec_val.size() / 2];
   return true;
 }
 
-
 /**
 * @brief Display to standard output min, max, mean and median value of input range
 * @param begin start of range

src/openMVG/numeric/numeric_test.cpp

@@ -33,6 +33,7 @@
 #include "testing/testing.h"
 
 #include <iostream>
+#include <set>
 
 using namespace openMVG;
 using namespace std;
@@ -121,6 +122,26 @@ TEST(Numeric, MeanAndVarianceAlongRows) {
   EXPECT_NEAR(1.25, variance(1), 1e-8);
 }
 
+TEST(Numeric, minMaxMeanMedian)
+{
+  const int vec_size = 12;
+  std::vector<double> values(vec_size);
+  std::iota(values.begin(), values.end(), 0.0);
+  double min, max, mean, median;
+  minMaxMeanMedian(values.cbegin(), values.cend(), min, max, mean, median);
+
+  // For the GT value, use a set to sort the value
+  //  and then collect the min, max, median and mean value
+  const std::set<double> set_values(values.cbegin(), values.cend());
+  EXPECT_NEAR(0.0, min, 1e-8);
+  EXPECT_NEAR(values.size() - 1, max, 1e-8);
+  auto set_begin_it = set_values.cbegin();
+  std::advance (set_begin_it, values.size() / 2);
+  EXPECT_NEAR(*set_begin_it, median, 1e-8);
+  const double mean_gt = std::accumulate(values.begin(), values.end(), 0.0) / static_cast<double>(values.size());
+  EXPECT_NEAR(mean_gt, mean, 1e-8);
+}
+
 /* ************************************************************************* */
 int main() { TestResult tr; return TestRegistry::runAllTests(tr);}
 /* ************************************************************************* */