feat(discretization): added implementation of checked_block_size

tho see the note in the header, this could be made more general likeley

src/lib/serif/discretization/blocks/system_layout.cpp

@@ -0,0 +1,20 @@
+#include "serif/discretization/blocks/system_layout.hpp"
+
+#include <limits>
+#include <stdexcept>
+#include "serif/utils/error/errors.hpp"
+
+namespace serif::discretization::blocks
+{
+    std::size_t checked_block_size(const std::span<const std::size_t> sizes)
+    {
+        using namespace utils::errors;
+
+        std::size_t total = 0;
+        for (const std::size_t size : sizes) {
+            serif_ensure<SERiFErrorCode::INVALID_DOF_COUNT, std::overflow_error>(size <= std::numeric_limits<std::size_t>::max() - total, "The system's packed coefficient count overflows size_t. Check the individual coordinate sizes.");
+            total += size;
+        }
+        return total;
+    }
+} // namespace serif::discretization::blocks