Write copy_ptw test, and it works!

Had to add in a bit more zeroing of the main header in the initial write of the
ptw file since the amount of space that Altair reserves for itself and actually
uses is different, but it worked after that.

Occasional bytes in the outputs may be different, since they're not actually
used in the header, so while this toolkit writes zeroes, Altair just leaves
whatever happened to be there already.