0.2.1

This release fixes three wrong answers. None of them raised an error. A model
that meets one builds a different row, or reads a different value, in 0.2.1.
No file, archive or import breaks.

• A sum over a variable with a where:, beside a parameter over fewer
  dimensions, such as sum(y + w, over=f) with w over f alone. The
  parameter now drops out where the variable is absent, in the constraint row
  and in evaluate. Before, it stayed in, so the row had a wrong right-hand
  side.
• A divisor, a power's base or an exponent that is a sum(...). It is now
  taken as its total, in a build and in a read. Before, it was taken summand by
  summand.
• A reported quotient whose divisor is absent or zero. It now has no value at
  that coordinate. Before, it read inf or raised a DataError.

Result.dual now states the sign of a dual: the rate at which the optimal
objective rises with the row's right side, for every comparator, under either
sense and on every solver. No value changes. mathspec's reference now defines
dual(c) the same way
(energy-models/mathspec#751).

The pull requests since 0.2.0:

• fix(engine): a sum leaves out a parameter where a variable added to it is absent, in the model and in a read (#1784)
• fix(engine): a divisor, a power's base or an exponent that is a sum is taken as its total rather than summand by summand (#1779)
• fix(api): a reported quotient is absent where its divisor is absent or zero (#1776)
• docs(api): a dual is the rate at which the optimal objective rises with its row's right side, on every sink (#1774)