Made Cell classes serializable

AbstractCell and ArrayDataAccess now implement Serializable, to
facilitate Cell serialization to disk.

Several fields in AbstractCell had to be made non-final, and a
zero-param constructor was added.
Merge branch 'cell-serialization'

This branch makes ArrayDataAccess and AbstractCell appropriate for
serialization.

Both now implement the Serializable interface. AbstractCell no longer
has "final" fields - as these need to be populated by reflection. Also
added a zero-parameter constructor to AbstractCell.

No change in Cell performance on the PerformanceBenchmark was observed.