Refactoring of cloud-metric routines

[leifdenby]

This issue tracks the progress of refactoring of all cloud metric functions. Each function should be callable with the signature cloudmetrics.cloud_metric_name(da_mask, optional_argument=default_value, ...). Each metric should have a set of tests to ensure it is implemented correctly. There will be a separate pull-request for each metric.

• cf.py Refactored cloud fraction #31
• computeMetrics.py - removed
• cop.py Convective Organisation Potential (COP) refactor #41
• createDataFrame.py - removed
• csd.py Refactor cloud size distribution (CSD) #44
• cth.py add masked statistical reductions #46
• cwp.py add masked statistical reductions #46
• fourier.py Refactoring Fourier metrics #30
• fracDim.py Refactoring fractal dimension #29
• iorg.py Refactor iorg #21
• iorgPoisson.py incorporated into iorg with a function argument to pick the reference distribution (refactor iorg for object-labels #54)
• network.py Will defer incorporating network metrics until we know a little more about what they're good at.
• objects.py Refactoring cloud object scales metrics #23
• openSky.py Refactoring of open sky metric #22
• orientation.py Refactoring orientation metric #24
• rdf.py Refactor Radial Distribution Function (RDF) #43
• scai.py Refactor Simple Convective Aggregation Index (SCAI) #42
• twpVar.py - removed
• utils.py - removed
• woi.py Refactoring wavelet organisation indices (woi) #32

[leifdenby]

Changed functionality:

• "cloud objects" code previously excluded cloud objects smaller than a threshold size. This will be moved to parent function so that this parameter can be varied