Feature/rearrange tests

The purpose of this PR is to divide tests into smaller groups to e.g. make possible running smaller chunks of tests more conviniently.

For now I moved only distance and set ops. Each algorithm has its own directory but since set operations uses the common code I moved this code to the set_ops directory. There are also some helper *.cpp files there running e.g. all set_ops tests for L/L or P/P combinations. So now it looks like this:

    algorithms
        + set_ops
        + difference
        + intersection
        + ...

So I'm wondering, maybe it'd be better to move the set operations directories deeper e.g. to set_ops directory? And also create a "fake" directories linear_linear and pl_pl and put only Jamfiles in there. Like this:

    algorithms
        +set_ops
            + difference
            + intersection
            + ll
            + pp
            + ...

Then it'd be possible to run all of the setops tests, the tests of one algorithm or the tests of all algorithms for L/L or P/P combination. And those helper *cpp files wouldn't be needed, plus it'd be possible to run required tests from bjam level.

Btw, which directory name do you like more "set_ops" or "setops", "rel_ops" or relops?