All courses: Change path to exercise folder

[jeeemil]

All courses
This issue is actually related to all courses, but I would suggest that we start with the most common QGIS courses. This issue is closely related to issue #23

Description
When creating tailored courses with CourseCrafter, there's often an issue that path to the layers added to the exercise is not correct.

For example

Lisää projektiin seuraavat aineistot:

..kurssihakemisto/4. Harjoitus/TieViiva.shp

In tailored course the exercise number could be something else.

Solution
Go through the exercises and check the data paths. Change it to something like

Lisää projektiin seuraavat aineistot:

TieViiva.shp

I think we can be confident enough that our participants can find the correct layer without the full path. When creating a tailored course you still need to create your own data folder, but this way we minimize tedious work when tailoring courses.

This is issue could be useful to split it up for each course.