Enhance direct unit tests for basic and private read functions

[oruebel]

This is a follow-up to #85

• Add unit test for src/Utils.cpp , mergePaths() function (See Add unit tests for Utils.hpp #137 )
• Add unit tests for new private HDF5IO internal functions. These may not require separate dedicated tested, but may be covered via the tests of the public core interfaces functions. However, we should check that they are indeed covered. NOTE: I checked with Cline and added another test in Add unit tests for BaseIO::findTypes #139
  • HDF5IO:: readDataHelper (note, there are two variants of this function)
  • HDF5IO::readStringDataHelper (note there are two variants of this function)
  • HDF5IO::getAttribute
• Add unit test for BaseIO::findTypes (possibly as part of the HDF5IO tests) (See Add unit tests for BaseIO::findTypes #139)
• Check unit test coverage for RegisteredType. This should be largely covered by the other tests, but we should check. (See Add unit tests for BaseIO::findTypes #139)
• Check unit test coverage for ReadIO. This should be largely covered by the other tests, but we should check.