plot_l1() function for producing automatic coefficient plots

[cmbosancianu]

Function would end up taking the output from l1_reg(), and use it for generating a coefficient plot.

The most challenging part of this is to figure out how to do faceting, in case the number of groups is very large, e.g. 80 or over. In this case, an automatic variable should be created, which splits the rows in k equal chunks, and this variable is used for faceting.

[cmbosancianu]

Still thinking what the best way to tackle this function is:

• one alternative is to make it a straightforward plotting function, which just takes in a data set of coefficients and produces a plot.
• the other one is to let the function take in a data set of observations and a model, and then the function passes this to l1_reg() and gets back the data frame of coefficients for the plot. Currently, this is the approach used, as it makes it most convenient for the user.

The challenge with this second one is that it has to take in all arguments that l1_reg() needs, as well as some additional arguments that refer only to the plot. That makes it a bit cumbersome, even though sensible defaults are currently selected.