Use integers to represent media indices

[fangq]

Currently, we use an unsigned char to represent a label of a medium label. Because the highest bit of the char is used to represent the detector label, so, the max number of tissues mcx can represent is only 2^7=128.

Since there is no storage benefit to use a char variable inside the cuda kernel because of byte alignment, it probably would not add any overhead, if not faster, to use an integer array to represent the domain.

If we expand the label to 32bit, we can use a subset of bits for special masking to speed up some of the calculations.