Add more information about tree branch-length mode based on fossil calibrations?

[jimallman]

@LunaSare is interested in a clear indication of whether a tree's branch lengths are based fossil calibration, and whether it's a primary or secondary calibration.

The curation app has several options for a tree’s branch length mode:

• Expected number of changes per site
• Expected number of changes
• Time
• Bootstrap values
• Posterior support values
• Other (describe)

Time requires ‘Time unit’, so we could add a checkbox: ‘[X] Based on fossil calibration’?

OR add a mode ‘Time based on fossil calibration’, expects ‘Time unit’ ...and maybe expects ‘Primary calibration’ vs. ‘Secondary calibration’?

[jimallman]

Related question for @LunaSare: What is your intention for this change? Are we hoping that these settings can be read and used in synthesis? If so, we should use distinct and machine-readable values, as opposed to 'Other (describe)'.