Adding some basic printing

[zanellia]

Adding basic printing. Setting arg->printlevel = 1 the progress towards the solution can be monitored.

[giaf]

I would add a makefile flag to use C pre-processing to remove all printing dependencies, like in BLASFEO https://github.com/giaf/blasfeo/blob/master/Makefile.rule#L86 such that it is possible to compile the library without any printf in the code.

And I would do the same for all 3 QP solvers: dense, ocp and tree_ocp