cmake issue when nesting mrob into other project

[g-ferrer-sk]

paths are absolute, lets check this one.

[g-ferrer-sk]

not just solving the problem of paths when nesting the project into a different project, but in general cmake needs some cleaning to be more understandable.

[anastasiia-kornilova]

Some clean ups are provided in #76

[g-ferrer-sk]

still it is not tested what happens when we include mrob into another project.