An integer HDF5 datatype (i.e. one that has class `H5T_INTEGER`) can be
signed or unsigned. This commit adds a class `IntegerType` which
supports the method `isSigned`.

Intended use:

    auto dtype = /* obtain datatype */;
    if(dtype.getClass() == DataTypeClass::Integer) {
      auto int_type = dtype.asIntegerType();
      if(int_type.isSigned()) {
        // ...
      }