ENH: Add 6sV version reporting/checking to output and tests

[dmwelch]

• Fixes erroneous compile instructions that cause test failures
• Updates pip_requirements to include pandas

[robintw]

Thanks.

I've merged this with a few modifications:

1. I've taken pandas out of the requirements file as it is an optional dependency and is only used for one, fairly specialised, piece of functionality (importing AERONET data). Pandas can be a pain to install for people who aren't using a 'scientific Python distribution' and so keeping it out of the requirements list was a conscious decision.
2. Rather than taking out the note about flags in older versions of GCC I've altered it to say that some people have found that this works, but some have found it doesn't - I know some people on the mailing list have said it solved problems for them.
3. I've modified your change to test.py so that the tests pass: SIXSVERSION wasn't defined in that file.

Thanks again.