Add an element index option to EXPECT commands

[alan-baker]

Right now the EXPECT command uses IDX which is a byte-offset. It would likely be useful to measure in terms of the buffer's data type. So ELEM_IDX of 5 of an int32 buffer would be the 6'th int32. This will be useful with STD140 layouts.

I would expect that data type of the EXPECT command must match the data type of the buffer to use ELEM_IDX.