Allow virtual function for DEFINE_FIELD

[oruebel]

See discussion at #85 (comment)

Currently DEFINE_FIELD in RegisteredType.h creates a templated member function. However, when "redefining" a field in a child class B with DEFINE_FIELD the existing member function that was inherited from the base class A is being "hidden" instead of "override". This is fine if we are using a pointer of type B. However, if we cast the pointer to the base class A, then the definition of the function from A is being used since the member function is not virtual. However, C++ does not allow virtual templated functions.

Here the corresponding note in the docs (which should be updated when this issue is being addressed):

aqnwb/docs/pages/devdocs/registered_types.dox

Lines 82 to 87 in 7b88122

	* \note
	* ``DEFINE_FIELD`` creates templated, non-virtual read functions. This means if we
	* want to "redefine" a field in a child class by calling ``DEFINE_FIELD`` again, then
	* the function will be "hidden" instead of "override". This is important to remember
	* when casting a pointer to a base type, as in this case the implementation from the
	* base type will be used since the function created by ``DEFINE_FIELD`` is not virtual.