Refactored value formatting according to issue 52; also refactored how time axis autoformatting works in the plotting package.
Update timeutils.removeleapyears() to allow a different "leap day" to be omitted.

Also, allow multiple "leap days" to be omitted, and multiple non-"leap days" as well.

Allow casting to other time axes, like ModelTime360.
Clean up dangling reference to ModelTime365.