Changed the way the heatmap title is made

Before it used ax.set_xlabel(), but in testing I found that the xlabel is difficult to find and it would be much simpler to use the ax.set_title() function and orient it to the bottom of the graph.
Created unit tests for the plot() method

Tests check the xtick and ytick labels, and the sanity of the output, regarding the title and amount of axes, and whether or not it returns a Figure. Also I was unable to find a way to retreive information regarding the cmap input. Help would be much appreciated.