API/ENH: Add option to magfig() to overplot detrended data on raw data

[KCollins]

Description

• Add another flag to magfig (is_raw_shown = False, e.g.)
• Plot original data (non-detrended) with lighter line width/dotted line
• Overplot detrended data
• Verify all flag configurations

Potential impact

This should make it easier to see the impact of detrending the data.