clean percent response variables down to one (or two) values as needed

[lizzieinvancouver]

We need to know:

• what % of studies have the time course?
• what is common in EGRET?
• what is common in USDA?

[lizzieinvancouver]

@dbuona @FrederikBaumgarten @christophe-rd worked on USDA:

• mean germination time (time to 50% germination of the total that germinate)
• final germination (in percent) will be common for USDA and usable!
• germination rate is germination percentage over a fixed (often unknown) period of time.
• germination energy and germination capacity but we should probably look at predictor variables for USDA too ... and put together a definition of the terms (check out also cleanrespvar.R for some notes within if we do create a master list). And figure out how often rate or energy or capacity co-occur for the same species with percent. @dbuona start a new issue or two for this!

@DeirdreLoughnan @wangxm-forest worked on EGRET ...

[dbuona]

@dbuona see how often germination percentage covaries with other germination metrics