Simplify serialization of DataSet objects

vtkm::cont::DataSet is a dynamic object that can hold cell sets and fields of many different types, none of which are known until runtime. This causes a problem with serialization, which has to know what type to compile the serialization for, particularly when unserializing the type at the receiving end. The original implementation "solved" the problem by creating a secondary wrapper object that was templated on types of field arrays and cell sets that might be serialized. This is not a great solution as it punts the problem to algorithm developers.

This problem has been completely solved for fields, as it is possible to serialize most types of arrays without knowing their type now. You still need to iterate over every possible CellSet type, but there are not that many CellSets that are practically encountered. Thus, there is now a direct implementation of Serialization for DataSet that covers all the data types you are likely to encounter.

The old SerializableDataSet has been deprecated. In the unlikely event an algorithm needs to transfer a non-standard type of CellSet (such as a permuted cell set), it can use the replacement DataSetWithCellSetTypes, which just specifies the cell set types.

Fixes #725 (closed)