Calibration plotting tweaks

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
