Viewer, 3D Animation Export, Lib Namespace

Changes from release 0.2:

New options for controlling the viewer window:

• ctrl-W (or command-W on MacOS) closes the window.
• The HOME key or R key resets the camera to its home position. (Ivo Cavalcante)
• -Olazy : Redraw only on user input. Disables animation & FPS counter.
  This reduces GPU power consumption on a laptop. (Ivo Cavalcante)
• -Obg=colour sets the background colour (default is white).
  Use any Curv colour expression, eg -Obg=black.
• -Oaa=supersampling_factor : enables spatial antialiasing.
  Try 2, 3 or 4, or 1 to disable. This is expensive and slow, but looks nice.
• -v : show GLSL shader compile time & object size
• docs: https://github.com/doug-moen/curv/blob/master/docs/Viewer.rst

Other changes to the 3D viewer window:

• You can zoom out further before clipping occurs (Ivo Cavalcante).
• You can zoom in closer before clipping occurs.

New options for PNG export:

• Export of 3D shapes now possible.
• -v : log verbose output to stderr
• -Obg=colour : set background colour
• -Oaa=supersampling_factor : spatial antialiasing, defaults to '4'
• -Oanimate=duration_in_seconds : export an image sequence.
  Used to create animated GIFs or video files from an animated shape.
• -Ofstart=frame_start_time
• -Ofdur=frame_duration, used with -Oanimate or -Otaa
• -Otaa=supersampling_factor : temporal antialiasing
• PNG export now uses RGB format, instead of RGBA format. Smaller files.
• docs: https://github.com/doug-moen/curv/blob/master/docs/Image_Export.rst

Language changes:

• frac n returns the fractional part of n.
• The hyperbolic functions: sinh cosh tanh asinh acosh atanh.
• lib: a hierarchical module namespace for the standard library.
• lib.web_colour: standard CSS colour names. Eg, lib.web_colour.peach_puff.
• file import now supports directory syntax:
  https://github.com/doug-moen/curv/blob/master/docs/language/File_Import.rst
  A prerequisite for lib and the future package manager.

Other changes:

• Makefile supports make -j4 for parallel compilation (Ivo Cavalcante)
• in -Oname=value, values are now Curv expressions.
• Improved error messages.
• Improved --help text. curv --help -o format now documents the -O options
  supported by the specified output format.
• Various minor bug fixes.

I would like to thank Ivo Cavalcante for his contributions to this release.