make OPF automatically supply missing gencost

[rdzman]

If a user tries to run an OPF with a case that does not define the gencost matrix, it would be nice if MATPOWER (warned and) created a default gencost that used equal linear costs to minimize system active power losses.

It would also make sense to add an option opf.objective that would allow the user to request loss minimization instead of using gencost even when it is provided. There may be other "auto-generated" objective options that could be useful as well.