Font scaling in labels of plot.matrix2d()

[petergthatsme]

This issue is meant as a unitaryHACK bounty issue.
Bounty: $50

Function plot.matrix2d() is used when creating matrix element plots (it is called by qubit methods <qubit>.plot_matrixelements()). It has an argument show_numbers which explicitly adds text labels to indicate matrix element sizes. This example below shows how that works:

tmon2 = scq.Transmon(
    EJ=30.02,
    EC=1.2,
    ng=0.0,
    ncut=31
)
tmon.plot_matrixelements('cos_phi_operator', evals_count=10, show3d=False, show_numbers=True);

When too many levels are included, however, these labels start overlapping with each other, as in:

tmon.plot_matrixelements('cos_phi_operator', evals_count=16, show3d=False, show_numbers=True);

We are looking for a small enhancement that adds simple logic to plot.matrix2d() which would scale the font size so that the text labels can be easily distinguished (within reason).
Ideally this modification should take into account the figure size, so that if more levels are being included, but the figure size is large enough, then the font used does not have to be extremely small.