This function creates a matrix where columns 1 and 2 correspond to tip labels and column 3 gives the depth of the MRCA of that pair of tips.
It is strongly based on `treeVec`

and is used by `relatedTreeDist`

and `treeConcordance`

where tip labels belong to "categories".

`tipsMRCAdepths(tree)`

- tree
An object of class phylo

```
tree <- rtree(10)
plot(tree)
tipsMRCAdepths(tree)
#> tip1 tip2 rootdist
#> [1,] "t1" "t10" "0"
#> [2,] "t1" "t2" "1"
#> [3,] "t1" "t3" "4"
#> [4,] "t1" "t4" "3"
#> [5,] "t1" "t5" "3"
#> [6,] "t1" "t6" "2"
#> [7,] "t1" "t7" "2"
#> [8,] "t1" "t8" "3"
#> [9,] "t1" "t9" "1"
#> [10,] "t10" "t2" "0"
#> [11,] "t10" "t3" "0"
#> [12,] "t10" "t4" "0"
#> [13,] "t10" "t5" "0"
#> [14,] "t10" "t6" "0"
#> [15,] "t10" "t7" "0"
#> [16,] "t10" "t8" "0"
#> [17,] "t10" "t9" "0"
#> [18,] "t2" "t3" "1"
#> [19,] "t2" "t4" "1"
#> [20,] "t2" "t5" "1"
#> [21,] "t2" "t6" "1"
#> [22,] "t2" "t7" "1"
#> [23,] "t2" "t8" "1"
#> [24,] "t2" "t9" "2"
#> [25,] "t3" "t4" "3"
#> [26,] "t3" "t5" "3"
#> [27,] "t3" "t6" "2"
#> [28,] "t3" "t7" "2"
#> [29,] "t3" "t8" "3"
#> [30,] "t3" "t9" "1"
#> [31,] "t4" "t5" "5"
#> [32,] "t4" "t6" "2"
#> [33,] "t4" "t7" "2"
#> [34,] "t4" "t8" "4"
#> [35,] "t4" "t9" "1"
#> [36,] "t5" "t6" "2"
#> [37,] "t5" "t7" "2"
#> [38,] "t5" "t8" "4"
#> [39,] "t5" "t9" "1"
#> [40,] "t6" "t7" "3"
#> [41,] "t6" "t8" "2"
#> [42,] "t6" "t9" "1"
#> [43,] "t7" "t8" "2"
#> [44,] "t7" "t9" "1"
#> [45,] "t8" "t9" "1"
```