add tidy() method for model_fit objects

[malcolmbarrett]

Closes #112

This PR adds a tidy() method for model_fit objects. Right now, it uses a tryCatch to grab the generics message (which seems about the right message to me). I could always remove that and let the generic handle it, since it's a pretty readable message on its own. Currently, the tidy.model_fit object is exported, which is a bit out of norm, so I can change that if needed.

library(tidymodels)
library(parsnip)

logistic_reg() %>%
  set_engine("glm") %>%
  fit(Class ~ funded_amnt + int_rate, data = lending_club) %>%
  tidy(conf.int = TRUE, exponentiate = TRUE)
#> # A tibble: 3 x 7
#>   term        estimate  std.error statistic   p.value conf.low conf.high
#>   <chr>          <dbl>      <dbl>     <dbl>     <dbl>    <dbl>     <dbl>
#> 1 (Intercept)  169.    0.157         32.7   3.54e-235  125.      231.   
#> 2 funded_amnt    1.00  0.00000515     0.537 5.91e-  1    1.000     1.00 
#> 3 int_rate       0.853 0.00846      -18.7   1.99e- 78    0.839     0.868

rand_forest(mode = "classification") %>%
  set_engine("randomForest") %>%
  fit(Class ~ funded_amnt + int_rate, data = lending_club) %>%
  tidy()
#> Error: No tidy method for objects of class randomForest

[topepo]

The new tibble seems to have a version conflict for rlang. Can you change the description file to require rlang (>= 0.3.1 ) ?

[topepo]

The example fails with:

> logistic_reg() %>%
+   set_engine("glm") %>%
+   fit(Class ~ funded_amnt + int_rate, data = lending_club) %>%
+   # tidying model object and passing arguments to broom:::tidy.glm
+   tidy(conf.int = TRUE, exponentiate = TRUE)
Error in confint.glm(x, level = conf.level) : 
  package 'MASS' must be installed
Calls: %>% ... suppressMessages -> withCallingHandlers -> <Anonymous> -> confint.glm

Is there something else that you could demo with so that we wouldn't have to add MASS as a dependency?

[malcolmbarrett]

Wow, this is a buried one. stats:::confint.glm depends on MASS, so let me try removing the conf.int argument here

[malcolmbarrett]

Oh, a follow-up issue: tidy()is in generics, but tidy.glm() is in broom. Putting it in suggests will still fail with a documentation example, right? I could add to depends, but that's a big one, too. Any way to have an example and avoid this?

[topepo]

Oy vey. Can we do without an example? I'm trying to keep the dependencies low and broom would blow that up (even in suggests; travis times out).

[malcolmbarrett]

Yeah, it's a big one, so I get that! I also feel like the example is nice but not essential given the context, so I removed it

[codecov-io]

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[topepo]

Thanks!

[github-actions]

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