Add spell check

[gowerc]

We currently have no spell checking anywhere in the package or testing. Would be good to have something put in place

[luwidmer]

@gowerc I started a branch for this. Some decisions still need to be made:

• Do we fail the spell check test in case of errors? I would be in favor.
• Currently it's skipped on CRAN. I'm also in favor of that.
• MNAR and NMAR are both used - this seems inconsistent?

[gowerc]

Do we fail the spell check test in case of errors? I would be in favor.

Yes I think it would be good to have a failing test for spelling as otherwise I would likely forgot 😅

Currently it's skipped on CRAN. I'm also in favor of that.

Definitely skip on CRAN 🙃

MNAR and NMAR are both used - this seems inconsistent?

I think it should just be MNAR, @gravesti / @wolbersm do you have any preference ?

[wolbersm]

AFAIK both MNAR and NMAR are used in the literature and mean the same thing ("missing not at random") but MNAR is more common.
Agree to just use MNAR. Thanks!