Add a function to compute differences of smooths

[gavinsimpson]

Compute differences of smooths for factor by smooths for pairs of levels: difference_smooths().

Output should be a tibble

# A tibble: 1 x 7
  smooth by    level_1 level_2  diff    se     x
  <chr>  <chr> <chr>   <chr>   <dbl> <dbl> <dbl>
1 s(x)   f     lev1    lev2      0.2 0.002     1
....

Needs:

• difference_smooths()
• draw() method
• internal draw functions for different smooths
  • draw_1d_difference()
  • draw_2d_difference()
  • Implement draw_3d_difference() for difference_smooths() #256
  • Implement draw_4d_difference() for difference_smooths() #257
• Add methods for
  • gam()
  • bam()
  • gamm()
  • gamm4()

Example fits

data <- data_sim("eg4")
m <- gam(y ~ fac + s(x2, by = fac) + s(x0), data = data)