Save objective value of dispatch optimisation with output data

[alexdewar]

This is a scalar value that you get every time you run the optimisation. I'm not entirely sure what the significance of it is, but Adam has said it would be useful to have this number in the output data.

Unfortunately, there is currently no way to get this value using the highs crate (see issue), though it shouldn't be hard to add that functionality (the underlying C API does provide a function for this). We can hack around it with some nasty pointer shenanigans, but I'd rather not do this except as a last resort.

The objective value is printed out as part of the log output of the HiGHS library, but we have to enable this logging in order to see it. This is probably good enough for now.