added index_to_cubecoord and cubecoord_to_index

[towzeur]

Added two function

index_to_cubecoord : index to Cube coordinates (pointy) conversion
based only on its input (int index) and CubeCoord.directions

cubecoord_to_index : Cube coordinates (pointy) to index conversion
based only on its input (int x, int y, int z)

tested and verified

for (int i=0; i<37; i++){
    CubeCoord c = index_to_cubecoord(i);
    int j = cubecoord_to_index(c.x, c.y, c.z);
    System.out.println(String.format("%d -> (%d,%d,%d) -> %d", i, c.x, c.y, c.z, j));
    assert i == j;
 }

0 -> (0,0,0) -> 0
1 -> (1,-1,0) -> 1
2 -> (1,0,-1) -> 2
3 -> (0,1,-1) -> 3
4 -> (-1,1,0) -> 4
5 -> (-1,0,1) -> 5
6 -> (0,-1,1) -> 6
7 -> (2,-2,0) -> 7
8 -> (2,-1,-1) -> 8
9 -> (2,0,-2) -> 9
10 -> (1,1,-2) -> 10
11 -> (0,2,-2) -> 11
12 -> (-1,2,-1) -> 12
13 -> (-2,2,0) -> 13
14 -> (-2,1,1) -> 14
15 -> (-2,0,2) -> 15
16 -> (-1,-1,2) -> 16
17 -> (0,-2,2) -> 17
18 -> (1,-2,1) -> 18
19 -> (3,-3,0) -> 19
20 -> (3,-2,-1) -> 20
21 -> (3,-1,-2) -> 21
22 -> (3,0,-3) -> 22
23 -> (2,1,-3) -> 23
24 -> (1,2,-3) -> 24
25 -> (0,3,-3) -> 25
26 -> (-1,3,-2) -> 26
27 -> (-2,3,-1) -> 27
28 -> (-3,3,0) -> 28
29 -> (-3,2,1) -> 29
30 -> (-3,1,2) -> 30
31 -> (-3,0,3) -> 31
32 -> (-2,-1,3) -> 32
33 -> (-1,-2,3) -> 33
34 -> (0,-3,3) -> 34
35 -> (1,-3,2) -> 35
36 -> (2,-3,1) -> 36