Fix headers on example data files

[GenevieveBuckley]

Closes https://github.com/GrotjahnLab/surface_morphometrics/issues/9

I've fixed the problem with the headers in the example data file, using this method:

import mrcfile

filename = "TE1.mrc"

with mrcfile.open(filename, mode='r+', permissive=True) as mrc:
    print("Original mrc file header:", mrc.header)
    mrc.header.map = mrcfile.constants.MAP_ID
    mrc.update_header_from_data()
    print("Fixed mrc file header:", mrc.header)

...and now the files can be opened without needing the permissive=True keyword argument. There's no error or warning now.

with mrcfile.open(filename) as mrc:
    print(mrc.data.shape)

The updated files have been re-tarred, and uploaded here in this PR.

[GenevieveBuckley]

Possibly the README text should also be updated, in this section.

I'm not entirely sure what to write here though. I don't completely want to get rid of the note about the permissive keyword argument, since it seems likely other users will export from Amira and might want to know about it.

We could also leave it for now, since the instructions still work if you pass the permissive kwarg, regardless of the header format.

[GenevieveBuckley]

Update: I think I found a good way to explain it, and have edited the README now too.

[bbarad]

I like this solution!