Merge pull request #6 from abingham/master

PEP8 cleanup
Improves handling of mutable default arguments (empty dictionaries).
Removes unused legacy file.
Don't unnecessarily pass the filename around on the read-path.
Rename header and trace-header definition constant dictionaries to be more documentary, and in upper case.
More renaming towards PEP 8 standards.
Improved handling and tolerance of non-compliant SEG Y revisions.
More PEP 8 naming compliance
Minor formatting consistency.
Simplify code.