Technical debt

[GregoryPetrochenkov-NOAA]

This PR addresses all but three tasks in #128 .

Additions

• continuous_stat_funcs.rst
• continuous_statistics.rst
• schemas.rst: partial fix of task, perhaps changing docstrings will suffice
• continuous_statistics.py: Class to register functions and process continuous statistics

Changes

• CONTRIBUTING.MD SPHINX_CONTRIBUTING.MD: Separated README compilation as a new step
• compile_readme_and_arrange_docs.py: Change name
• Tutorial.ipynb SphinxTutorial.ipnyb: Change compute_metrics to compute_categorical_metrics
• statistics.rst: add new pages
• init.py: Add continuous processing class
• gval_array.py: Remove plotting function from dataarray only class
• gval_xarray: Add categorical and continuous plotting and changes to docstrings
• compute_comparison.py: Remove private members from docs
• continuous_metrics.py: Add continuous processing
• rasterize.py: Enforce attributes are not null or empty upon rasterization
• categorical_stat_funcs.py: Adjust return section in docs
• categorical_statistics.py: Remove references to private functions in docs
• continuous_stat_funcs.py: Remove superfluous arguments
• visualize.py: Accomodate categorical and continusous plotting as well as multiband and dataset types
• test_accessors.py cases_accessors.py: Add plotting tests
• test_compute_continuous_metrics.py case_compute_continuous_metrics.py: Add testing for computing continuous metrics
• test_continuous_metrics.py case_continuous_metrics.py: Add testing for continuous metrics