Add build requirements

[KristofferSkare]

As it is now one cannot use the tool purely by installing the requirements using pip.
This is mainly because the two requirements for building are missing, i.e conan and cmake.

It seems that conan can maybe just be added in the requirements.txt and it will work.
However, cmake might be a bit harder to setup.

We need to investigate this and make changes to requirements.txt and documentation with the solution we find.

[KristofferSkare]

This issue is followed up by #35.