Reverted semantics of assign_by.

The problem described in previous commit was caused only inside the
cell's copy constructor, where an assignemt for `std::unique_ptr` was
attempted. Since `std::unique_ptr`'s cannot be copy-assigned, I have
made that assignment manually moving the just cloned other's segment to
the cell being constructed. The original `assign_by` method, relying on
STL's `assign()` cannot accommodate this without changing its
semantics (as pointed out by @halfflat), which would then also conflict
with STL's semantics.

src/cell.hpp

@@ -90,8 +90,11 @@ public:
         spike_detectors_(other.spike_detectors_),
         probes_(other.probes_)
      {
-        util::assign_by(segments_, other.segments_,
-            [](const segment_ptr& p) { return p->clone(); });
+         // unique_ptr's cannot be copy constructed, do a manual assignment
+         auto siter = segments_.begin();
+         for (const auto& s : other.segments_) {
+             *siter = std::move(s->clone());
+         }
      }
 
     /// add a soma to the cell

src/util/rangeutil.hpp

@@ -62,37 +62,13 @@ Container& append(Container &c, const Seq& seq) {
 // Assign sequence to a container
 
 template <typename AssignableContainer, typename Seq>
-enable_if_t<
-    std::is_copy_constructible<typename AssignableContainer::value_type>::value,
-    AssignableContainer&
->
-assign(AssignableContainer& c, const Seq& seq) {
+AssignableContainer& assign(AssignableContainer& c, const Seq& seq) {
     auto canon = canonical_view(seq);
     c.assign(std::begin(canon), std::end(canon));
     return c;
 }
 
 
-// This version of assing is needed for assigning segment_ptr's (i.e.,
-// unique_ptr's) with Clang
-
-template<typename AssignableContainer, typename Seq>
-enable_if_t<
-    !std::is_copy_constructible<typename AssignableContainer::value_type>::value &&
-     std::is_move_constructible<typename AssignableContainer::value_type>::value,
-    AssignableContainer&
->
-assign(AssignableContainer& c, const Seq& seq)
-{
-    auto citer = c.begin();
-    for (auto s : canonical_view(seq)) {
-        *citer = std::move(s);
-    }
-
-    return c;
-}
-
-
 // Assign sequence to a container with transform `proj`
 
 template <typename AssignableContainer, typename Seq, typename Proj>