cjcampbell/isocat: isocoat 0.2.4

Optimize 'cumsumbelow' function

• Function deals with NA values more efficiently.
• When imported into makeCumSumSurface, function iterates faster over RasterLayers.
• Removed dependency on purrr::map and pipe operators within function.
• The purrr package removed from suggested packages.

• Modified conditional class check within cumsumAtSamplingLocation and clusterSimmatrix functions, in anticipation of change to matrix objects inheriting from class "array" (which broke old class check code).

• Vignette updated, examples improved, enable stage-skipping with working examples.

• Tweaked package title.
• Added 'additionalModel_name' argument to isotopeAssignmentModel.
• Added 'seealso' and 'alias' components to function documentation.
• Fixed typo in examples for makeQuantileSimulationSurface function.
• Clarified function description for schoenersD function.
• Tweaked wording in example for cumsumAtSamplingLocation function.