Use consistent input checking

[Deleetdk]

Right now input checks are chaotically written using a variety of methods. Some use my own functions, others use assertthat functions. It's better to reuse well-tested code, so switch to assertthat functions. Fill in with missing tests.

[Deleetdk]

Other options.

https://cran.r-project.org/web/packages/ensurer/vignettes/ensurer.html

[Deleetdk]

Actually I quite like the is_ function. Flexible. Can be made more flexible by adding the half equation, math function support. Maybe use a formula e.g. ~<2 is shorthand for x<2, where x is the variable.

[Deleetdk]

I deleted check_missing. Pointless function. All one has to do is call each of the arguments. If it's actually missing, R displays the standard missing argument error to the user.

[Deleetdk]

Neither package turned out to be useful for my purposes. assertthat has no easy way to make custom error messages, it has to be done in some complicated functional way. ensurer has the same problem, except it's worse.

So I rolled my own simple function: fail_if. It simply evaluates the input and throws an error with a custom message message if it is true.