Revise shaded areas for plots

[stefsmeets]

Sometimes showing the standard deviation for the distributions is confusing.

• [x] Add option to show fan / distribution of lines (e.g. like a box plot with lines at 0th, median, 100th percentiles, shaded between 25th / 75th percentiles)
• [x] Add option to show msd as shaded region

Affected plots:

• [x] msd_per_element
• [x] autocorrelation