Add support for mason jar and garden hose threads

[amatulic]

Changes to threading.scad:

• Added support for garden hose male and female couplers; new modules garden_hose_threaded_rod(), garden_hose_male(), garden_hose_female(), and garden_hose_gasket()
• Added module to generate a cylinder with consistent knurling types, now called by garden_hose_female() as well as all of the ...cap() modules in bottlecaps.scad
• Knurling types are now:
  • knurled (trunc_pyramids)
  • sharp_knurled (diamonds)
  • ribbed (trunc_ribs)
  • sharp_ribbed (ribs)
  • hex_faces (rounded nut shape)
• Added descriptions for several examples in trapezoidal_threaded_rod()
• Corrected documentation inconsistencies (e.g. default anchor in docs disagreed with actual default in code)
• Prefixed all assert messages with newlines
• Minor copyediting of docs for grammar and spelling

Changes to bottlecaps.scad:

• Added support for Mason jar necks and caps; new modules mason_neck() and mason_cap().
• Added some introductory text to some sections, giving brief descriptions of the standards
• Several module parameters had undocumented defaults; added these values to the docs
• Changed arbitrary default values in cap/neck adapter modules to use PCO1881 where applicable, and documented this
• Reviewed all anchor defaults for sensibility
• Prefixed all assert messages with newlines
• Minor copyediting of docs for grammar and spelling