Suppressed unused parameter warning

The function's arguments are mandated by HDF5
so this is likely genuinely meant to be unused.
Fixed some -Wsign-compare warnings

Two types:
1) Just added a cast. The signed value was confirmed non-negative a few lines earlier, so cast it to unsigned.
2) changed a for loop index variable from unsigned to signed by adding a new 'j' index variable different from the 'i' one.