Interfacing IpOpt from Crocoddyl

Adds acrobot example

- remove passing by const reference elementary types
- change `Jsum_dx` by `Jint_dx`
- control bounds now depend on whether they are defined
- remove unnecessary scopes
-change `2e19` by `std::numeric_limits`
- overwrite th_stop function wrapping `tol` parameter
- add preprocessor IPOPT guard where necessary
- fix unittest to not consider IPOPT if not installed
- add license in the files

- add noalias when computing derivatives
- remove unnecessary objects from the data class
- remove call to calc in derivatives computation methods

- express initial condition in its tangent space
- number of states and controls independnet for each node
- create ipopt data struct to store temporal solver data
- add specific setter to specify ipopt options

Initial condition constrain was defined as: `g(Dx_0) = x_0 \oplus Dx_0 = x(0)`.
Now is defined as `g(Dx_0) = x(0) \ominus (x_0 \oplus Dx_0) = 0`

- variable renaming
- use of `assert_pretty`
- add `namespace crocoddyl`
- use of `model&` and `data&` to access problem model and data
- remove ipopt legacy comments

- changed ipopt related variable names
- use of setCandidate() function

Added more unit-tests for contact/impulse cost residuals and removed useless set_id

Sync submodule cmake

Enabled OpenMP in Apple + improved documentation in solvers