Rich: add index to each PMT pixel by sorting translations inside its pmt volume

Assign each pmt pixel an index [0, 63] by sorting pixel volume translations inside its pmt volume.
This will result in a easy pixel numbering scheme (independent of pixel paths in the geometry -> version independant).
Neighbor pixels can now be easily determined and e.g. crosstalk implemented (following soon).

Row major, view upstream.

	x→
↓y	0	1	2	3	4	5	6	7
	8	9	10	11	12	13	14	15
	16	17	18	19	20	21	22	23
	24	25	26	27	28	29	30	31
	32	33	34	35	36	37	38	39
	40	41	42	43	44	45	46	47
	48	49	50	51	52	53	54	55
	56	57	58	59	60	61	62	63