Email file memory fix

[smclay]

When I ported the old code from using the rc822 module to using the email instead I did not realise that I had introduced a new problem. The old rf822 module was very memory efficient where as the new email modules appears to read the entire file contents into memory. I have written a simple file parer that only pareses the MIME file primary header unit. This is all we need for ALMA when trying to determine if the file is a multipart/mixed or multipart/related.

[coveralls]

Coverage increased (+0.06%) to 69.606% when pulling 444574c on smclay:email-file-memory-fix into e7fc383 on ICRAR:master.

[smclay]

@rtobar looks like a problem in the python 3.7 build environment

[smclay]

@rtobar the travis CI fails for python 3.7 only. I do not know why

[rtobar]

The problem with 3.7 seems like a Travis environment issue that has been communicated to the setuptools people: pypa/setuptools#3118. I think we can safely ignore this for the time being, I'll pin the setuptools version later on the master branch to avoid this issue.

As for the code changes, they only touch ALMA-related code. If you're happy then I'm happy; in other words, I'll merge straight away.