Nmpyc

Fixed an issue that occurs during simulations for timvariant contiuous time problems.

New release of nmpyc fixing known issues of the previous version.

The changes compared to the previous version include the following points:

- fixed errors in model.solve_ocp()
- fixed errors in the dicretization methods for LQPs
- fixed small bugs in the opti class
- chnaged handling of the time parameter t when using casadi optimizer

**First official release of nmpyc.**

The changes compared to the previous pre-release version include the following points:
- changed return value of t_cl, x_cl, u_cl to numpy array (matching the documentation)
- minor changes in the string outputs
- revision of the doc files and the README.md

First public release of the nmpyc Python package.