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Features:
- date fields are allowed to contain values stored with leading spaces
instead of leading zeroes.
- dbf.header returns field definition objects if accessed as a list
or as a dictionary.
- added raw data access methods: DbfRecord.rawFromStream(),
DbfFieldDef.rawFromRecord().
- added conversion error handling: if ignoreErrors=True is passed
with Dbf constructor arguments, then failing field value conversions
will not raise errors but instead will return special object
INVALID_VALUE which is equal to None, empty string and zero.
.ignoreErrors property of the Dbf instances may be toggled also
after instance initialization.