Calibration plotting tweaks

[bas-rustenburg]

Tweaks for calibration plots suggested by @jchodera

• make sure the figure width is set to one column (3.5 inches?)
• reduce the axis label font size
• increase the axis tick labels to at least 6pt
• Use log bias weight $g$ (kT) for the y-axis (no capitalization)
• Use update for the x-axis (no capitalization)
• Option to add a legend that indicates which state index is which color