refactoring, some bug fixes and increased test coverage

[jsiedentop]

I've added some more lineString tests. There are two test cases where at least one is not behaving as expected:

I compare two lines with same start and end point but ...

• one line has a third point in the middle -> The lines are recognized as not equal.
• one line has the same end point twice -> The lines are recognized as equal.

I would expect, that booth cases are equal or not equal depending on the understanding of equality. Are the lines only equal, if they have exactly the same structure or is it enough if they have the same shape?

@lukas-h

[jsiedentop]

The questions have been solved during the refactoring. Comparing geometries with a different amount of points should always result in not being equal. The test cases in which the behavior deviated from expectations were simply bugs. Therefore, I am merging this to master so that we can release a new version for dartclub/turf_dart#174 soon.