Performance improvement of TileSet class by caching.

[HaitaoHuang-USDA]

We are adding caching in the TileSet class.

The performance test on Ceres shows the open tile process is greatly sped up if the requested tiles are already cached. This caching doesn't depend on file system caching.

Test results on Ceres.

n_tiles	rep_id	duration_1 (s)	duration_2 (s)
10	1	5.991	2e-05
10	2	0.0	2e-05
10	3	0.0	2e-05
10	4	0.0	2e-05
10	5	0.0	2e-05
20	1	3.602	8e-05
20	2	0.0	3e-05
20	3	0.0	3e-05
20	4	0.0	3e-05
20	5	0.0	3e-05
50	1	10.522	8e-05
50	2	0.0	6e-05
50	3	0.0	6e-05
50	4	0.0	6e-05
50	5	0.0	6e-05
100	1	18.068	0.00015
100	2	0.0	0.00038
100	3	0.0	0.00013
100	4	0.0	0.00013
100	5	0.0	0.00016
200	1	39.097	0.00033
200	2	0.0	0.00024
200	3	0.0	0.00023
200	4	0.0	0.00024
200	5	0.0	0.00024
500	1	123.412	0.00085
500	2	0.001	0.00064
500	3	0.001	0.00067
500	4	0.001	0.00066
500	5	0.001	0.00065
1000	1	204.596	0.00157
1000	2	0.002	0.0012
1000	3	0.001	0.00121
1000	4	0.001	0.00125
1000	5	0.001	0.00121