Ticks rendering refactor

Here is the refactor of calculateTicksCoordinates per the discussions had on GitHub issues.

It took me more time than I had originally expected as I had to brush up on some Algebra concepts and figure out the way to properly address the challenges of:
* obtaining the proper coordinates for zero, 
* establishing the ticks boundaries, taking into account the straight attributes, direction of the line if it is an axis, and the fact that the line's p1 and p2 might be out of the board, but still have a bit of the line that is visible and ticks should be rendered,
* obtaining the proper deltaX and deltaY
* generating the labels text
* etc.

I took the opportunity to do some clean up where required and I think the logic now is easier to follow.

The commit messages have more info.

I've tested the different scenarios we had discussed in #54: having an Axis, or relying on the anchor attribute to establish the direction and value of each tick. I also tested for different combination of points position (in and out of boundaries) and straight attributes to ensure that if a line was visible and ticks should be rendered, they got rendered with the appropriate values.

Now that I realise, I have not updated the relevant documentation about the anchor attribute to reflect the new changes, so it would be good to do so after merging the changes in.