ANOVA (added error message zero residual SS)

fixes #43
ANOVA (added error message inf values dep. var.)

fixes #114
ANOVA (added error message negative weights)

fixes #108
ANOVA (added basic plot function)

I started working on a function that makes profile plots in JASP for
the ANOVA function. However, because ggplot2 is not yet supported by
JASP, I commented out the function