Completes constexpr-enabling of morph::vec<>

docs/ref/coremaths/vec.md

@@ -37,7 +37,9 @@ Header file: [morph/vec.h](https://github.com/ABRG-Models/morphologica/blob/main
 
 `morph::vec` is a fixed-size mathematical vector class. It derives
 from `std::array` and can be used in much the same way as its STL
-parent. It has iterators and you can apply STL algorithms.
+parent. It has iterators and you can apply STL algorithms. It is
+constexpr-capable, meaning that it can be incorporated into constexpr
+functions to do compile-time maths.
 
 ```c++
 namespace morph {

morph/range.h

@@ -4,27 +4,27 @@
  * A tiny container class to hold the min and max of a range. I wanted a common type in
  * which to return minmax values for use in morph::vec and morph::vvec. An option would
  * have been an std::array, but I prefer this, as vvec doesn't otherwise need to include
- * <array>.
+ * <array>. range can be used in constexpr functions.
  *
  * Author: Seb James
  * Date: Sept. 2023.
  */
 
-#include <string>
-#include <sstream>
 #include <ostream>
+#include <limits>
 
 namespace morph {
 
     // Forward declare the class and stream operator
     template <typename T> struct range;
     template <typename T> std::ostream& operator<< (std::ostream&, const range<T>&);
 
+    // range is a constexpr-friendly literal type
     template <typename T>
     struct range
     {
-        range() {}
-        range (const T& _min, const T& _max) : min(_min), max(_max) {}
+        constexpr range() {}
+        constexpr range (const T& _min, const T& _max) : min(_min), max(_max) {}
 
         // The minimum
         T min = T{0};
@@ -41,38 +41,31 @@ namespace morph {
         // r.search_init();
         // for (auto d : data) { r.update (d); }
         // std::cout << "The range of values in data was: " << r << std::endl;
-        void search_init()
+        constexpr void search_init()
         {
             this->min = std::numeric_limits<T>::max();
             this->max = std::numeric_limits<T>::lowest();
         }
 
         // Extend the range to include the given datum
-        void update (const T& d)
+        constexpr void update (const T& d)
         {
             this->min = d < this->min ? d : this->min;
             this->max = d > this->max ? d : this->max;
         }
 
         // Does the range include v?
-        bool includes (const T& v) { return (v <= this->max && v >= this->min); }
-
-        // Output a string with notation "[min, max]" to indicate a closed interval
-        std::string str() const
-        {
-            std::stringstream ss;
-            ss << "[" << this->min << ", " << this->max << "]";
-            return ss.str();
-        }
+        constexpr bool includes (const T& v) { return (v <= this->max && v >= this->min); }
 
         // Overload the stream output operator
         friend std::ostream& operator<< <T> (std::ostream& os, const range<T>& r);
     };
 
+    // Output a string with notation "[min, max]" to indicate a closed interval
     template <typename T>
     std::ostream& operator<< (std::ostream& os, const range<T>& r)
     {
-        os << r.str();
+        os << "[" << r.min << ", " << r.max << "]";
         return os;
     }