Pretty printing for `Counts` and `Probabilities`

[kahaaga]

In #341, I remove the dependency on DimensionalData.jl. The result is that we lose pretty printing of the marginals (outcomes) for Counts and Probabilities. We should implement pretty printing manually here.

[kahaaga]

By modifying the DimensionalData.jl printing code, I'm getting close to something here.

julia> Counts([50, 44, 33], ["cake", "meat", "veggie"])
 Counts{Int64,1} over 3 outcomes
 "cake"    50
 "meat"    44
 "veggie"  33

julia> Counts(rand(1:3, 5, 2))
 5×2 Counts{Int64,2}
                          Outcome(1)  Outcome(2)
 Outcome(1)                        3           1
 Outcome(2)                        3           1
 Outcome(3)                        3           1
 Outcome(4)                        1           1
 Outcome(5)                        3           3

julia> Counts(rand(1:10, 4), ["o1", "o2", 56, 2])
 Counts{Int64,1} over 4 outcomes
   "o1"   5
   "o2"   5
 56       8
  2      10

julia> c = Counts(rand(1:30, 2, 3, 3), (['a', 'e'], 2:2:6, [(1, 2), (2, 1), (3, 1)]))
 2×3×3 Counts{Int64,3}
[:, :, 1]
                 2   4   6
 'a'            15  18  27
 'e'            23  13  17
[and 2 more slices...]

I just need to get rid of the extra spaces that sometimes occur, and I think this is good to go.