MonotoneCubic paths feature completion

[rosco54]

New Pull Request

The monotoneCubic paths feature is now complete to the same level as 'straight' and 'smooth' paths for chart types: Line, Area, RangeArea:

• Handling of null data points.
• Forecast data points with rangeArea series. In actuality, an issue with series of different lengths, nothing to do with forecastDataPoints but was excluded by explicit condition unnecessarily.

Refactored the createPaths() monotoneCubic branch. Several other tidy-up fixes to enable this to work.

Fix libs/monotone-cubic.js splice.slice():
Was crashing on single point slices.
NEEDS TO BE PUSHED UPSTREAM.

New sample to verify most of the above. The rangeArea series have embedded nulls (at start, end and middle) plus forecast data points that were an issue previously for monotoneCubic.

The initial aim was to fix the following sample chart that broke at version 3.45.1 (shown fixed):

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes