Added hxl.model.HXLDataset class to hold an in-memory HXL dataset
(if desired), and hxl.parser.readHXL() and hxl.parser.writeHXL() to
read/write the dataset.
Add documentation for readHXL and writeHXL.
Start adding schema support.
Completed single-value tests (still need to support row).
Make HXLColumn.sourceColumnNumber and HXLColumn.columnNumber optional.
Added row-occurrence validation (HXLSchemaRule.validateRow)
Started implementing HXLSchema class, to apply all rules to each row.
Add a default value to HXLRow.get()
Deal with missing or short text header row.
Add function to load a schema from a HXL dataset.
Start adding hxlvalidate script.
Start to separate scripts out for installation.
Fix indentation.
Fixed up hxl2geojson command.
Minor script refactoring.
Update usage info.
Refactor to use argparse.
Refactor comments and formatting in scripts.
Update script examples.
Start roughing in error reporting.