Command line parsing refactored to sub-parser usage

Subcommand: bbox - Current default
Subcommand: gpx - read bounding box from gpx file (currently placeholder)
Move command line parsing into function
Basic GPX file reading and bounding box calculation

Still missing margin, and failure with xmlns handling
More consistent usage of class Coord
Add support of xz-compressed GPX files and Change to Python3

Test is still outstanding
Assume namespace http://www.topografix.com/GPX/1/1 for GPX XML reader
How can I ignore the namespace?

Also added minor fix and GPX reader assertion
Ignore XML namespace (xmlns) during gpx file read

Thanks to Stack Overflow for code snippet
Add margin to boundaries extracted from gpx trkpt's

Thanks to Stack Overflow for Code Snippet
Update README for GPX file reader