Integrate anova_table() and summarize_clusters() as a method of generic R funcitons

[wleoncio]

About anova_table()

anova() is a generic R function with several methods. Currently (lsasim 2.0.2.9004), the anova_table() function is defined independently. It would be great if it were instead implemented as a method of anova() so that one could call the latter instead of the former. The steps to do this are rather straightforward:

• [x] Give the output of cluster_gen its own S3 class (e.g. lsasim-cluster)
• [x] Rename anova_table() as anova.lsasim-cluster
• [x] Add unit tests?
• [x] Rename print_anova_table()? No, because it's internal.

About summarize_clusters()

The same thing could be done for summarize_clusters(), which could be a method of summary().

Summary

• [x] Convert anova_table() into a method of anova()
• [x] Convert summarize_clusters() into a method of summary

[wleoncio]

This reference on S3 classes might be useful: http://adv-r.had.co.nz/S3.html