Closes #1872. Implemented to_basic for geom_function

[moutikabdessabour]

@cpsievert Implemented to_basic for GeomFunction class.

[cpsievert]

Great, thanks, please add a test to demonstrate it works

[moutikabdessabour]

@cpsievert by test do you mean just running ggplotly(p) without the error being thrown or a visual test?

[cpsievert]

A visual test (expect_doppelganger())

[moutikabdessabour]

@cpsievert I'm sorry to bother you again. but if you can post some resources where I can read more about expect_doppelganger|_built I would appreciate it. Do you save the output as svg in the _snaps folder and then pass the name of the svg to expect_doppleganger?

[cpsievert]

There are lots of examples under test/testthat, for example https://github.com/plotly/plotly.R/blob/master/tests/testthat/test-plotly-area.R

[moutikabdessabour]

@cpsievert thanks I found the function in helpers-vdiff and was able to understand the way it functions. I added two unit tests.