Value checking

[nickrsan]

Numerous spots in the code don't currently check their parameters. We should basically audit the whole user-facing portion for this (which is small) and make sure we check that values are in-bounds. Numerous other enhancements are blocked by this, so this needs to happen first

[nickrsan]

We'll want to do this in two layers - type hinting for us and our linters and actual checking of parameter values that users pass - this will include removing some bad defaults I currently have set so that users have to pass values, then checking things like, for paths, that the path is a string (or Path object) and that the path exists, etc

[AdamJCrawford]

@nickrsan what do you want changed before marking this as complete? Side note about checking paths, as far as I know, best practice is to not check if a path is valid and then use it because of the potential race condition where something happened to that path after you checked it but before accessing it. Rather you should wrap all the stuff that deals with files in try/except.