Change arrows colours and formats

[jonsv89]

Choose colours that can be differentiated easily, right now there are cases where blue and red arrows are hardly differentiated.
Choose different types of arrows (dashed, solid...) to make visually easy to detect the interactions that have been previously described by epidemiological networks.

[jmfernandez]

This is a work in progress. Latest commits include different arrow shapes to better distinguish inverse from direct comorbidities. Also, the lower relative risk values in both cases are not similar any more