Add support for xsd('auto-date')

[laurianvm]

consider making the | xsd filter a bit smarter:
create xsd( "auto-date"), to only deliberate between optional accurancy date variants (datetime, date, month-year, year, …)

(copying from IMIS-1598)

[mpo-vliz]

some upfront thoughts / while at it:

• additional auto-number could also negotiate between int and float
• auto-any could do over anything available

strategy to follow imho is trying to be as specific as possible and then drop down from there?

flow for auto-date:

• type datetime
• type date
• string parseable to datetime
• string parseable to date
• string matching -?YYYY-MM for gyearmonth
• string matching -?YYYY for gyear
• int for gyear
• error

flow for auto-number

• type float / double
• type int
• string parseable as int
• string parseable as float
• error

flow for auto-any:

• type boolean
• type float/double
• type int
• type datetime
• type date
• string is validuri -> anyURI
• string parseable to datetime
• string parseable to date
• string matching -?YYYY-MM for gyearmonth
• string matching -?YYYY for gyear
• string parseable as int
• string parseable as float
• string exactly true or false (ignoring case) for bool
• string version of the content

generally getting a bit worried that this is stepping away of the more rigid fail-fast approach we had up to now
this might lead to undesired side-effects and silently ignored problems to be discovered late ...

[mpo-vliz]

fix provided in #133
took the liberty to make some re-ordering of the fallback steps vs what I mentioned above...