Reduce redundancy in moment_all functions

The if-else-blocks in the moment_all functions couldn't be replaced with
the optval function, because this would lead to poor performance. If the
center variable is present, the calculation of the mean can be skipped,
but the optval function prevents this optimisation.

Author: Carl Burkert

src/stdlib_stats_moment_all.fypp

@@ -22,6 +22,7 @@ contains
         ${t1}$ :: res
 
         real(${k1}$) :: n
+        ${t1}$ :: center_
 
         if (.not.optval(mask, .true.)) then
           res = ieee_value(1._${k1}$, ieee_quiet_nan)
@@ -31,10 +32,11 @@ contains
         n = real(size(x, kind = int64), ${k1}$)
 
         if (present(center)) then
-         res = sum((x - center)**order) / n
+          center_ = center
         else
-         res = sum((x - mean(x))**order) / n
+          center_ = mean(x)
         end if
+        res = sum((x - center_)**order) / n
 
       end function ${RName}$
     #:endfor
@@ -52,6 +54,7 @@ contains
         real(dp) :: res
 
         real(dp) :: n
+        real(dp) :: center_
 
         if (.not.optval(mask, .true.)) then
           res = ieee_value(1._dp, ieee_quiet_nan)
@@ -61,10 +64,11 @@ contains
         n = real(size(x, kind = int64), dp)
 
         if (present(center)) then
-         res = sum((real(x, dp) - center)**order) / n
+          center_ = center
         else
-         res = sum((real(x, dp) - mean(x))**order) / n
+          center_ = mean(x)
         end if
+        res = sum((real(x, dp) - center_)**order) / n
 
       end function ${RName}$
     #:endfor
@@ -82,14 +86,16 @@ contains
         ${t1}$ :: res
 
         real(${k1}$) :: n
+        ${t1}$ :: center_
 
         n = real(count(mask, kind = int64), ${k1}$)
 
         if (present(center)) then
-         res = sum((x - center)**order, mask) / n
+          center_ = center
         else
-         res = sum((x - mean(x, mask))**order, mask) / n
+          center_ = mean(x, mask)
         end if
+        res = sum((x - center_)**order, mask) / n
 
       end function ${RName}$
     #:endfor
@@ -107,14 +113,16 @@ contains
         real(dp) :: res
 
         real(dp) :: n
+        real(dp) :: center_
 
         n = real(count(mask, kind = int64), dp)
 
         if (present(center)) then
-         res = sum((real(x, dp) - center)**order, mask) / n
+          center_ = center
         else
-         res = sum((real(x, dp) - mean(x,mask))**order, mask) / n
+          center_ = mean(x, mask)
         end if
+        res = sum((real(x, dp) - center_)**order, mask) / n
 
       end function ${RName}$
     #:endfor