add full support for triclinic cells to dcd & xtc.

• implement coordinate unwrapping for xtc and dcd and remove test that inhibits using it.
• correct the system cell info for both cases (two different bugs: dcd had wrong lengths, xtc would always have an orthogonal cell).
• always apply scaling factor in xtc output for wrapped and unwrapped cases.
• full documentation of cell information in dcd.