Update to StatModel

[lgatto]

Hi @statOmics @ococrook - I inadvertently pushed to statOmics/msqrob2 instead of lgatto/msqrob2 to then send a PR. So here's a short summary of the two commits:

1. StatModel() now returns a valid empty model (threw an error before)

   > StatModel()
   Object of class "StatModel"
   The type is "fitError"
   There number of elements is 0 

2. A column of StatModels in a DataFrame now displays as expected (threw an error before)

   > df
   DataFrame with 2 rows and 2 columns
         x          mods
   <integer>        <list>
   1         1 StatModel:rlm
   2         2  StatModel:lm

3. Update the man page.
4. Added a unit test.

Please discuss any additional updates that need to be done and/or close this issue if you are happy.

[statOmics]

Seems excellent to me!