future work for MIQCQP support

[rdzman]

Here is a short list of items that would be nice to include (eventually) for MIQCQP support.

• Support for more solvers (CPLEX, MOSEK, Artelys Knitro?)
  • miqcqps_cplex()
  • miqcqps_knitro()
  • miqcqps_mosek()
• Support for running a pricing stage after the mixed integer solve, in order to compute shadow prices (like the miqps_<solver> functions do).
  • With skip_prices and price_stage_warn_tol option support.
• Support for mip_gap and mip_gap_abs options.
• Support for mip_gap field in output.
• Make miqcqps_master() call miqps_master() explicitly for a problem with no quadratic constraints (I think).
  • Same for qcqps_master() and qps_master().