Moves most functions to AbstractCoverage; First impl of ViewCoverage

Migrates almost all functions to AbstractCoverage for reusability and simplicity

First implementation of ViewCoverage - parameter filtering

Conflicts:
	coverage_model/coverage.py
Fixes minor bug with examples.ptypescov()
Makes first pass at ComplexCoverage

First ComplexCoverage implementation for ComplexCoverageType.PARAMETRIC
Fixes all str checks to use basestring
Fixes test group for param_func tests
Updates to ComplexCoverage

Adds loading support

Renames ViewPersistenceLayer -> SimplePersistenceLayer and utilizes for
both ComplexCoverage and ViewCoverages
Adds ComplexCoverage example for SBE_37IM

Example creates 2 SimplexCoverages (Time, Lat, Lon) and (Time, L0 params) and a ComplexCoverage that references both and provides the L1 & L2 params
Updates SimplePersistenceLayer with data bounds
Merge pull request #39 from blazetopher/refactor_coverage

Refactor coverage