Add tests

[thokall]

As the package is now more complicated it would be could to add some tests to avoid creating problems whene altering code.
From the top of my head I suggest the following:

• Test that input data (irrespective of file type supplied generate identical dataframes with data. The same goes for the single data point entry
• Test that all distance functions generates the same number of perfect matches
• Check that thet GPS and date conversions are consistent over input.

There are lots of information on howto implement tests using testthat.

https://r-pkgs.org/tests.html