Smoothing

[LiKao]

This PR adds a smoothing function (by default gaussian smoothing) as in here.

I am currently in the process of implementing time-continuous multiple-regression (see e.g. here) in R and I am hoping it is possible to integrate this in the mousetrap package, as this seems to be the first go-to location for many researchers. In case this is possible, this would be the first in a series of PRs over the next days.

If this is not possible, I will just try to put this into a separate package depending on mousetrap.

I also added unit testing using the testthat framework for the new functions, as I wanted to ensure the smoothing behaves as expected in all regards. If no unit testing is desired for this project, just let me know and I will create a new PR where I keep this confined to my private repo.

[PascalKieslich]

Hi there,
adding TCMR to mousetrap is a great idea. Thanks a lot for your effort! I am happy to take a look at your functions in your pull request and then merge it into mousetrap. However, I may take a few days for this (background: I transitioned into an industry position some time ago - I am still maintaining and developing mousetrap, but have a bit less time for it at the moment).
I also think it is a great idea to add public unit testing to mousetrap. I was planning to do this for some time now, at least for the core functions and would add this after integrating your tests.
So again thanks for contributing this - and I will get back to you in a few days.
Best regards,
Pascal

[LiKao]

Ok, good to know. I will then submit some additional PRs to complete the functionality. I will keep the separate parts of TCMR in separate (but possibly dependent) PRs so you can get an overview of the different building blocks and their interactions.

[LiKao]

I just found some problems with the smoothing function when the track starts or ends with NA (e.g. angle data). I will add a commit fixing this shortly.