Tweak flax plant properties

[caethan]

• Should probably produce seeds rather than grains (check still grindable into flour)
• Adjust commonness/etc.
• Adjust size and nutritive values
• Look up potential herbal properties?

[caethan]

Been experimenting with plant properties and how they work, as well as reading up on flax.

• Seeds instead of grains is OK (can still grind for flour).
• Some reading on flax suggests that seed and fiber production are mutually exclusive.
• Flax is planted very densely - 40-50 plants per square seems about right.
• Sowing in April-May, ~50 days of growth, ~25 days of flowering, ~35 days of maturing before harvest is about right.
• Modern yields are about 1000 lbs/acre, or about a quarter pound per square meter. In-game, max yields of rye and barley are about 5.5 lbs per square (4 sq meters), or about 1.3 lbs per square meter. That should help me tune the total weight of seeds to produce.
• Flax grows best in standard fertile soils --- particularly not in clayey or marshy soils (so the standard grove/meadow/settlement is probably best).