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.

• [x] cf.py https://github.com/cloudsci/cloudmetrics/pull/31
• computeMetrics.py - removed
• [x] cop.py https://github.com/cloudsci/cloudmetrics/pull/41
• createDataFrame.py - removed
• [ ] csd.py https://github.com/cloudsci/cloudmetrics/pull/44
• [x] cth.py https://github.com/cloudsci/cloudmetrics/pull/46
• [x] cwp.py https://github.com/cloudsci/cloudmetrics/pull/46
• [x] fourier.py https://github.com/cloudsci/cloudmetrics/pull/30
• [x] fracDim.py https://github.com/cloudsci/cloudmetrics/pull/29
• [x] iorg.py https://github.com/cloudsci/cloudmetrics/pull/21
• [ ] iorgPoisson.py incorporated into iorg with a function argument to pick the reference distribution (https://github.com/cloudsci/cloudmetrics/pull/54)
• [ ] network.py Will defer incorporating network metrics until we know a little more about what they're good at.
• [x] objects.py https://github.com/cloudsci/cloudmetrics/pull/23
• [x] openSky.py https://github.com/cloudsci/cloudmetrics/pull/22
• [x] orientation.py https://github.com/cloudsci/cloudmetrics/pull/24
• [ ] rdf.py https://github.com/cloudsci/cloudmetrics/pull/43
• [x] scai.py https://github.com/cloudsci/cloudmetrics/pull/42
• twpVar.py - removed
• utils.py - removed
• [x] woi.py https://github.com/cloudsci/cloudmetrics/pull/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