Various fixes

Kinda a grab-bag of different fixes. I can split them out if needed, but was feeling too lazy for the moment...

* Make it possible to use SDXF as an importable folder i.e straight "import SDXF" using submodules rather than copying sdxf.py elsewhere
* Remove default layer from Drawing, as it's not used elsewhere, which stops output having an unneeded PYDXF layer
* Fix a minor spelling mistake in comments