Add get_quadratic_constraint and fix getting of quadratic function

[odow]

@joaquimg I'm not sure what the story is with Xpress but I needed these changes for Gurobi.

In particular, Gurobi counts linear rows separately from quadratic rows :(

The docs also say that get_quadratic_constraint returns 1-indexed columns. We probably need to check that this happens everywhere?

[codecov-io]

Codecov Report

Merging #24 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master      #24   +/-   ##
=======================================
  Coverage   56.21%   56.21%           
=======================================
  Files          13       13           
  Lines        1062     1062           
=======================================
  Hits          597      597           
  Misses        465      465

Impacted Files	Coverage Δ
src/solver_interface.jl	50% <ø> (ø)	⬆️
src/constraints/scalaraffine.jl	74.38% <0%> (ø)	⬆️
src/constraints/scalarquadratic.jl	0% <0%> (ø)	⬆️

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[joaquimg]

Those look good! I will modify xpress