median: add pure statement

Author: jvdp1

src/stdlib_stats.fypp

@@ -313,7 +313,7 @@ module stdlib_stats
     #:for k1, t1, o1 in IR_KINDS_TYPES_OUTPUT
       #:for rank in RANKS
         #:set RName = rname("median_all",rank, t1, k1, o1)
-        module function ${RName}$ (x, mask) result(res)
+        pure module function ${RName}$ (x, mask) result(res)
           ${t1}$, intent(in) :: x${ranksuffix(rank)}$
           logical, intent(in), optional :: mask
           real(${o1}$) :: res

src/stdlib_stats_median.fypp

@@ -16,7 +16,7 @@ contains
   #:for k1, t1, o1 in IR_KINDS_TYPES_OUTPUT
     #:for rank in RANKS
       #:set RName = rname("median_all",rank, t1, k1, o1)
-      module function ${RName}$ (x, mask) result(res)
+      pure module function ${RName}$ (x, mask) result(res)
         ${t1}$, intent(in) :: x${ranksuffix(rank)}$
         logical, intent(in), optional :: mask
         real(${o1}$) :: res