Add Cbc_modifyCoefficient to C interface.

[jetuk]

As discussed in #655.

I can do the OSI one too if you like?

[CLAassistant]

All committers have signed the CLA.

[jjhforrest]

Does it help you go faster?

Why not do Osi as well

[jetuk]

I can not immediately see how this can be achieved through the OSI. I admit I am not familiar with it though.

[jjhforrest]

Wait to see if anyone wants to do this from Osi C interface. If so, I can help.

[tkralphs]

I'm not really clear on what Osi C interface we're talking about and why we're waiting for that to be modified first. This looks reasonable to add just as it is.

[jjhforrest]

Modified Cbc_C to add modifyCoefficient. As it calls solver_->modifyCoefficient then Osi needs modifying - done

[jetuk]

So, this can be closed?

[jjhforrest]

Yes - if it works for you.

[jetuk]

Thanks for making the change!