Fixed false complete status when there are no Production Types
Added migrations.  Constrained wind_direction to 0-360
Added constraints for PDF fields based on this email:

N, m, d, s are all animal (head) counts and binomial, hypergeometric, neg binomial and Bernoulli are discrete distributions (so use integers).

Pareto is a continuous dis’n so floating point ok, as long as >0.

P is a probability, so constrained [0,1], and floating point.
Fixed a required foreignkey field
Merge remote-tracking branch 'origin/master'
Basic fix for #28 non-editable fields.
Fixed #28.  FixedSelect now displays an h4 label.
Improved layout and "add" buttons for Disease Spread.  Fixed #38.  Flaw: Required pairs remain editable.