MATLAB interface for Manual external cost in C

[mindThomas]

Updated MATLAB interface to add support for custom external C files for the cost.
Cost function names are source files are manually specified and compiled and linked during the solver compilation step.

[mindThomas]

Hey @FreyJo
Take a look again now. I did some modifications to get rid of the multiple C files at once.
Furthermore I also found a few bugs, including the one in generate_c_code_ext_cost.m and I also added support for a mix of Casadi and generic cost (e.g. the terminal cost being supplied as Casadi while the general stage cost is supplied as a generic C function).

[FreyJo]

Hey @mindThomas ,
I had a closer look.
Thanks for the bug fixes!
I would suggest you merge these changes mindThomas#3
then maybe @giaf has another look, and we merge this PR here.

[FreyJo]

Ah, I realized you @mindThomas allowed me to push on your branch, so I just did.

[mindThomas]

@FreyJo Yup. Since you are a maintainer of the upstream repository this permission is easily and if I am not mistaken by default granted when creating the pull request: https://docs.github.com/en/free-pro-team@latest/github/collaborating-with-issues-and-pull-requests/allowing-changes-to-a-pull-request-branch-created-from-a-fork

Thanks for the code formatting cleanup and the missed template.
test_template_ocp_linear_dae is however still failing in Travis.

[FreyJo]

Great! I just wasn't aware this is possible by default.
The last failing test was unrelated to this PR and fixed in #652