Added import statements to the file

This way we only need to load the file we need. At the moment some properties use invariants (and some invariants might use properties), so we always need to load both files to make everything work.
New solution to import problem. This version works even if the files are not in the same directory as Sage is running in
Merge branch 'master' of https://github.com/math1um/objects-invariants-properties

Conflicts:
	graphproperties.py
Fixed bug in lovasz_theta

There is no guarantee that the edges are integers from 0 to n-1
Merge branch 'master' of https://github.com/math1um/objects-invariants-properties