concentration of water in soil and sublcasses

[marieALaporte]

fixes #390

In this PR we created the terms:

• concentration of water in soil
• field capacity
• permanent wilting point
• water-holding capacity

[pbuttigieg]

Content curation report:

• concentration of water in soil - autogenerated. In pattern.
• field capacity - A complex concept. Adding qualifying comments. Added drained upper limit as exact synonym.
• permanent wilting point - Definition seems incomplete and grammatically iffy. Fixed and elaborated upon. as above.
• water-holding capacity - this is a disposition, rather than a concentration. Moved and redefined as such. The label was also made more specific (other things have water holding capacity). Created a more generic superclass.

@marieALaporte ready for your review.

[pbuttigieg]

@marieALaporte To clean up some other PRs, I'm merging this in.

If you need revision of any of the content, please open another PR. It should actually be easier when the others from your project are merged in too.