Read columns via DynamicTable

[oruebel]

#85 adds core read capabilities. For DynamicTable specifically, it would be useful to have convenience methods to read VectorData columns that may also be specified dynamically and not be part of the schema.

This is also related to #131 , which may partially address this issue or at least provide functionality for implementing a solution to this issue.

Currently, a user can get the path from DynamicTable and the colNames to construct the paths or use findTypes to search for columns in the DynamicTable. However, DynamicTable itself does not provide a way to create ReadDataWrapper objects for the VectorData columns it manages.

[oruebel]

#142 adds the RegisteredType::readField method which supports reading of arbitrary subfields. To make this even more convenient we could add a DEFINE_CHILD_FIELD function to read fields that are itself instances of RegisteredType which would return the specific pointer type for the container (e.g., VectorData) instead of returning the generic RegisteredType pointer.