Workaround for Juwels link error. (#747)

Fixes #746.

* Make `one_probe` an `inline` function rather than `static inline` in order to work around a linker issue (see issue #746).

arbor/include/arbor/sampling.hpp

@@ -12,7 +12,7 @@ using cell_member_predicate = std::function<bool (cell_member_type)>;
 
 static cell_member_predicate all_probes = [](cell_member_type pid) { return true; };
 
-static inline cell_member_predicate one_probe(cell_member_type pid) {
+inline cell_member_predicate one_probe(cell_member_type pid) {
     return [pid](cell_member_type x) { return pid==x; };
 }