Add a convenient API to retrieve the variable sizes

[masoud-najafi]

Similar to fmi3GetNumberOfEventIndicators and fmi3GetNumberOfContinuousStates, it would be very convenient to have an API to retrieve the size of a multi-dimensional variable whose size changes as a function of a structural variable.
an API such as
fmiStatus fmi3GetSizeofVariable(fmi3Instance instance, fmi3ValueReference valueReference, int dimension, size_t *size);
For example for a 2D variable dimension can be 0 or 1.
Of course, the size can be computed using the information inside the XML, but an API is much easier on run-time.