change ptw_get_frame to void **
added the save info to the test
started swapping endianness of read header if littleendian
finished swapping endianness of header on read
finished swapping endianness of header on read
Merge branch 'write_data' of github.com:jakelishman/ptw-tools into write_data

Conflicts:
	src/io.c
changed erroneous 1 to a 2 in a swap_endian call
Merge branch 'write_data' of github.com:jakelishman/ptw-tools into write_data
make ptw_get_frame return (void *) and add ability to read float values
updated get_data test to test float values too
made print_frame in get_data test know decide between float and int instead of having two different functions
swapped big- and little-endian definitions around because I'm an idiot
update warnings in Makefile
change ptw_get_frame to always use floats rather than mixed type
moved endianness swap in data read to earlier to avoid messing up cast to float
minor silly optimisations that don't affect speed all that much anyway
Fix ptw_get_data not skipping over irrelevant frame data.

Before, ptw_get_data would only skip based on the main header size and the frame
header size, but due to incompetence it didn't actually skip over unwanted data,
so it would return incorrect data for any frame != 0.
add comments to procrastinate actually doing the next step
Begin to add frame writing to ptw_file_write

Begin to write the logic which allows ptw_file_write to write frames.  At the
moment it's very broken, because the function is only passed one frame, but it
acts as though it's been passed many, so it just writes the same frame over and
over again.
Initialise tests/copy_ptw.c

This will eventually copy one ptw file to another by interpreting it, rather
than by cp or cat > or some other method.  Does nothing at the moment.