Fixed LatexFormatter with MultIndex index
Used `MultiIndex.formatter()` to correctly sparsify the index for `to_latex()`.

I was having an issue with printing Latex tables. This can be seen in the comparison of `to_string()` and `to_latex()` for the following table. Notice that the first row of `Vol8` should not be missing any labels because it marks a new label in the outermost level. This is correct in `to_string` and incorrect in `to_latex`.

Output of `to_string()`:
```
Volunteer Visit Stim  Marker             
Vol6      2     Long  CD137+           96
                      IFNg+           192
                Short CD154+           96
          14    Long  IFNg+           288
                Short CD137+          192
Vol7      2     Long  CD69-            96
                      IFNg+            96
                Short CD137+           96
          4     Short CD137+          192
                      IFNg+            96
          14    Short CD137+           96
                      CD154+           96
                      IFNg+            96
Vol8      2     Short CD137+          288
          4     Long  IFNg+           192
```

Output of `to_latex()`:
```
\begin{tabular}{llllr}
\toprule
     &    &      &        &  Total cells \\
Volunteer & Visit & Stim & Marker &              \\
\midrule
Vol6 & 2  & Long & CD137+ &           96 \\
     &    &      & IFNg+ &          192 \\
     &    & Short & CD154+ &           96 \\
     & 14 & Long & IFNg+ &          288 \\
     &    & Short & CD137+ &          192 \\
Vol7 & 2  & Long & CD69- &           96 \\
     &    &      & IFNg+ &           96 \\
     &    & Short & CD137+ &           96 \\
     & 4  &      &        &          192 \\
     &    &      & IFNg+ &           96 \\
     & 14 &      & CD137+ &           96 \\
     &    &      & CD154+ &           96 \\
     &    &      & IFNg+ &           96 \\
Vol8 & 2  &      & CD137+ &          288 \\
     & 4  & Long & IFNg+ &          192 \\
\bottomrule
\end{tabular}
```

I've tested the relevant pieces of this code using this example, but I would be grateful for help with adding tests to the repository. I'm not set up to develop the `pandas` codebase currently and am not exactly sure how to add the right tests in the right places. Thanks!

 - [x] Addresses issue #15480 
 - [x] tests added / passed
 - [ ] passes ``git diff upstream/master | flake8 --diff``