Libhxl

- improve support for private datasets: fix custom HTTP headers
with CKAN URL munging; add HXL_HTTP_HEADER and --http-header
support for all scripts
- add hxl.io.HXLIOException as base class for all HXL I/O-related
exceptions, with an optional url property
- add hxl.io.HXLAuthorizationException, and throw it for private
CKAN datasets or any 403 Forbidden response
- better error reporting for external validation errors

- report external validation errors (e.g. missing taxonomy)
separately (and don't invalidate the dataset)
- improve date parsing, and fix bug in borderline case (month but
no year)
- stability improvements for formulas (including "NaN" result for DIV0)
- efficiency improvements in validation tests
- fix a bug when reading an empty dataset
- fix a DIV0 bug in validation
- fix lexer conflict with jsonpath_rw package

- rewrite, correct, and simplify formula parsing
- fix bug in add_column filter
- add round() function to formulas

- fix setup.py to include new hxl.formulas package in the dist files (oops!)

- add spreadsheet-style formulas calculated from a row's
contents: https://github.com/HXLStandard/hxl-proxy/wiki/Row-formulas
- update add-column filter to support formula substitutions
- improve error reporting for private HDX resources
- fix bug with is min/is max aggregator and mixed datatypes
- fix date normalisation to preserve original value (and log an
error) if parsing fails
- bugfix for validation error when no row and/or column present

- added concat() aggregator for CountFilter
- added data_hash and columns_hash properties for
hxl.model.Dataset, together with hxlhash command-line script
- fixed parsing of YYYY-MM-DD hh:mm:ss SQL dates
- support opening zipped CSV files (thanks to Orest Dubay)