[T] Fix non-explicit qualified single param ctors.

Tests/MathLib/TestNonlinearNewton.cpp

@@ -29,7 +29,7 @@ template<class F_JACOBIAN>
 class ScalarDx
 {
 public:
-    ScalarDx(F_JACOBIAN &f_J) : _f_Jacobian(f_J) {}
+    explicit ScalarDx(F_JACOBIAN &f_J) : _f_Jacobian(f_J) {}
     // dx = - r/J
     void operator()(const double &x, const double &r, double &dx)
     {

Tests/MathLib/TestNonlinearPicard.cpp

@@ -51,7 +51,7 @@ public:
 class Example2
 {
 public:
-    Example2(std::size_t n) : A(n, n), b(n) {}
+    explicit Example2(std::size_t n) : A(n, n), b(n) {}
 
     void operator()(VectorType &x, VectorType &x_new)
     {

Tests/NumLib/TestODEInt.cpp

@@ -26,7 +26,7 @@ public:
     using LinearSolver = MathLib::LinearSolver<Matrix, Vector>;
     using NLSolver = NumLib::NonlinearSolver<Matrix, Vector, NLTag>;
 
-    TestOutput(const char* name)
+    explicit TestOutput(const char* name)
         : _file_name_part(name)
     {}