Plot versus distance along transect

[kakearney]

This issue is part of the JOSS review (openjournals/joss-reviews#2497)

For the section-plotting tools, you offer the option to place either latitude or longitude along the x-axis. Distance-along-transect would be another obvious candidate here, especially when section-plotting circuitous routes that may double back on themselves.

[lnferris]

Thanks for the suggestion! I have added this feature to bathymetry_section and general_section.