Add testing for each package function

[Paulj1989]

Problem

There is a very limited number of tests for any of the package functions. The lack of tests means it will be very easy to break the package without realising.

Proposed Solution

Testing! Testing! Testing! We need some robust tests for each function so that we can ensure the package is working as intended and identify any major issues before they get too far.

Additional Context

The Testing chapter in the R Packages book is definitely the best reference for this piece of work.

[Paulj1989]

• Visual Regression Testing & Graphical Differencing