ENH: using human-readable group names instead of integer ids in MultiComparison

In the current pip version, the output of line 145 in `statsmodels.examples.try_tukey_hsd.py` looks like this:

<pre>
Multiple Comparison of Means - Tukey HSD,FWER=0.05
============================================
group1 group2 meandiff  lower  upper  reject
--------------------------------------------
  0      1      1.5     0.3217 2.6783  True 
  0      2      1.0    -0.1783 2.1783 False 
  1      2      -0.5   -1.6783 0.6783 False 
--------------------------------------------
</pre>

Note the first and second column contain integer indices even though the `tukeyhsd` is passed a string array. This makes the table hard to read. With my changes, the output looks like this:

<pre>
Multiple Comparison of Means - Tukey HSD,FWER=0.05
===============================================
 group1  group2  meandiff  lower  upper  reject
-----------------------------------------------
medical  mental    1.5     0.3217 2.6783  True 
medical physical   1.0    -0.1783 2.1783 False 
 mental physical   -0.5   -1.6783 0.6783 False 
-----------------------------------------------
</pre>

I've applied the came change to `MultiComparison.allpairtest`