Improve unit test coverage for reading neurodata_type objects and fields

[oruebel]

This issue is a follow up to #85

Add unit tests for read. For every neurodata_type we should test that: a) the type itself can be read from file, and b) that all DEFINE_FIELDS specified for read can be read. As part of writing the tests, we should also double-check for each type, that all fields that AqNWB writes, that they are also exposed for read.

• TimeSeries, Data , VectorData, ElementIdentifiers, and Device are already covered by Add read tests for TimeSeries , VectorData, Data, ElementIdentifers, Device` #124
• ElectricalSeries (this is currently covered in the read example, but would be nice to have as dedicated tests)
• SpikeEventSeries
• NWBFile. NOTE: For reading the NWBFile container, there is the special case that NWBFile requires the path to the "/" so this may require some additional logic in the tests.

  aqnwb/src/nwb/NWBFile.cpp

  Lines 39 to 44 in bfa740a

  	NWBFile::NWBFile(const std::string& path, std::shared_ptr<IO::BaseIO> io)
  	: Container("/", io) // Always use "/" for the path
  	{
  	std::cerr << "NWBFile object is always the root. Path must be /" << std::endl;
  	assert(path == "/");
  	}

• ElectrodeGroup
• DynamicTable: Support reading of arbitrary RegisteredTypes  #143 adds method for reading columns by name
• ElectrodeTable : The following items have been completed in Support reading of arbitrary RegisteredTypes  #143
  • add DEFINE_FIELDS for default columns defined in the schema)
  • NOTE: ElectrodeTable is currently not a neurodata_type and uses REGISTER_SUBCLASS_WITH_TYPENAME. I.e., on read this will return a DynamicTable instead of an ElectrodeTable right now. See Support read with classes using REGISTER_SUBCLASS_WITH_TYPENAME #115
  • Add test for reading columns of the ElectrodeTable and read the ElectrodeTable itself
• ...