Present precip and snowfall in cumulative units

[rod-glover]

This PR resolves #173 by doing the following things:

• Revise the units configuration objects (units specs) to include labels (different than their identifiers) so distinct units specs (with distinct conversions) can present the same label. In particular, this is used for the cumulative precipitation units mm/year (cum), mm/season (cum), etc., which are all labelled mm but which are each scaled differently from the base units of kg m-2 d-1.
• Introduces and uses conversions from units spec shorthands to canonical units specs. All configuration objects now are (or should be) converted to canonical units specs.
• Merge the generic variables configuration item with display-specific variable configurations that depend on tab, season (annual, winter, ...), etc., and which include
  • display (absolute or relative anomaly values)
  • displayUnits (so that annual and winter precip values can be displayed with different cumulative units)
  • range, ticks (for maps and map colour bar)
• Raise up configuration in the maps component tree.
• Use more consistent naming for various objects.

This also goes partway to addressing #175 .

TODO:

• Restore low baseline messages (config processing issue).
• Add per-season configurations for other (non-precip) variables.
• Remove absolute precip from Summary.
• Set selector defaults back to normal.

[rod-glover]

Demo