Add operations module

[sgraaf]

Add operations module with editing and merging operations

• New operations module exposing the operations behind gpx edit and
  gpx merge as a public, reusable API (crop, trim, reverse,
  strip_metadata, reduce_precision, filter_points, merge), plus
  new split, simplify, smooth, shift_time and strip_extensions
  operations
• New convert_file() and detect_format() functions in the io
  module, exposing the operation behind gpx convert
• New gpx edit options: --split-time-gap, --split-distance-gap,
  --simplify, --smooth, --shift-time and --strip-extensions