Add a linspace-like function in AMReX_Algorithm.H (AMReX-Codes#3698)

## Summary

I would like to propose to add a linspace-like function in
AMReX_Algorithm.H , since filling a container with equally spaced
numbers is a task that occurs rather frequently. This PR proposes a
possible implementation.
The container is modified in place only if it has at least 2 elements. 

## Additional background

## Checklist

The proposed changes:
- [ ] fix a bug or incorrect behavior in AMReX
- [X] add new capabilities to AMReX
- [ ] changes answers in the test suite to more than roundoff level
- [ ] are likely to significantly affect the results of downstream AMReX
users
- [ ] include documentation in the code and/or rst files, if appropriate

Src/Base/AMReX_Algorithm.H

@@ -209,6 +209,24 @@ namespace amrex
         ))
     }
 
+    template<typename ItType, typename ValType,
+        typename std::enable_if<
+            std::is_floating_point<typename std::iterator_traits<ItType>::value_type>::value &&
+            std::is_floating_point<ValType>::value,
+        int>::type = 0>
+    AMREX_GPU_HOST_DEVICE
+    void linspace (ItType first, const ItType& last, const ValType& start, const ValType& stop)
+    {
+        const std::ptrdiff_t count = last-first;
+        if (count >= 2){
+            const auto delta = (stop - start)/(count - 1);
+            for (std::ptrdiff_t i = 0; i < count-1; ++i){
+                *(first++) = start + i*delta;
+            }
+            *first = stop;
+        }
+    }
+
 namespace detail {
 
 struct clzll_tag {};