Remove low-level functions from Callback

[jaeandersson]

Callback is supposed to be high-level interface to FunctionInternal. Working with work vectors is error prone and should reasonably be removed from any high-level interface.

The following methods are low-level and would be removed:

• virtual int eval(const double**, double**, casadi_int*, double*, void*) const;
• eval_sx
• alloc_w
• alloc_iw
• alloc_arg
• alloc_res

This would also resolve the current problematic infinite loop for CallbackInternal::eval and CallbackInternal::eval_dm (haven't looked deep into this, but the design doesn't look clean).