Only check isnan if applicable

[ianfiske]

Fix https://github.com/JuliaLang/Statistics.jl/issues/53

[ianfiske]

As per the discussion in #53, I added a check if isnan is applicable before calling it. Note that now, the tests fail because there is also no isfinite method for Millisecond. I can fix this with the same strategy of checking if the method is applicable, but this just feels clunky to me. I'm used to the "call the method" and it does the right thing for each type. Checking before calling so many methods just starts to look clunky, but if you think that's the right approach, I can easily add it.

[nalimilan]

I agree it's not ideal, but the only other solution I can see is to define isnan(::Any) and isfinite(::Any) to be false. Do you feel like proposing this in Base? That would probably allow making the code cleaner.

CI errors, but AFAICT it's no related to isfinite?

[nalimilan]

Bump.

[ianfiske]

@nalimilan Thanks for the bump, I've been busy with other things and I don't think I have time to make the PR to Base right now. Feel free to run with it if you have time before I do.