[sv-ec] soft constraint proposal

I have uploaded a new proposal for Mantis 2987. The new write-up incorporates all the feedback we've had, including the following:

- Makes soft a keyword.

- Uses the suggested "disable soft" instead of empty dist to reset soft constraints.

- Decouples the disable functionality from the soft-dist. Now the two features can be specified separately.

- Includes the BNF change from solve_before_primary to constraint_primary.

    Arturo

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