Bugfix: FlexYear::Historical broken with year ranges
Add BeforeParser
Add AfterParser
Remove obsolete initialisers from AfterParser and BeforeParser
Change after and before parsers to return nil instead of Float::INFINITY

Breaking change: Empty or invalid input returns nil instead of 0 for #year_low and #year_high
Fix Historical for @year_low or @year_high return nil
@base_year might not be set