Loading the vanadium to be used later on makes more sense to do at this
stage than at the start of the focusing step.

Also refactors the tests to focus on one path of execution per test.

Debug error appeared on windows when buttons are enabled at the end of
the calibration and focus.

By emitting a signal instead of calling the function to enable the
buttons directly, windows stops complaining about threads interacting
with the GUI.

Documentation has been added for the less obvious public and private methods.
Some of the methods in the model should be shown as private, so have
had methods refactored.

The calibration tab is now able to notify any tabs that subscribe to it what the current calibration is.
For now, this allows the focusing tab to perform it's algorithm based on the most recently loaded calibration.

Tests added for plotting functionality in the model and the current functionality of the presenter.