Resolve "test_files solution is temporary"

Closes #59 (closed)

I have added the toolbox.TestFiles repository as a submodule. I think this means that people can checkout wetb without getting all the test files.

To get the test files you need to git.exe submodule update --init

Test files are accessed via

filename = get_test_files("my_test_file.txt")

get_test_files will first look for <current folder>/test_files/my_test_file.txt and return the full filename. You can also specify the full path

If the file is too big run:

wetb.utils.test_files.move2test_files("my_test_file.txt")

which moves it to the same relative folder inside the ./WindEnergyToolbox/TestFiles/", and it will still be accessible via filename = get_test_files("my_test_file.txt") which now returns the new full filename as the old does not exist anymore