Implement date-time format auto-detection and casting

[ronaldtse]

Some of the specs fail like this:

  28) Reqif XML round-trip conversion with file strictdoc_02_read_reqif_input.reqif performs lossless round-trip conversion
      Failure/Error: expect(generated).to be_analogous_with(cleaned_xml_string)
      
        DIFF 1: expected node: <CREATION-TIME>2018-10-12T16:43:58+00:00</CREATION-TIME>
                actual node  : <CREATION-TIME>2018-10-12T16:43:58</CREATION-TIME>
                diff from    : 2018-10-12T16:43:58+00:00
                diff to      : 2018-10-12T16:43:58

Reason: date format had no time offset in input, but generated value has time offset

And

  16) Reqif XML round-trip conversion with file doors_export.xml performs lossless round-trip conversion
      Failure/Error: expect(generated).to be_analogous_with(cleaned_xml_string)
      
        DIFF 1: expected node: <CREATION-TIME>2014-07-09T10:19:00-04:00</CREATION-TIME>
                actual node  : <CREATION-TIME>2014-07-09T10:19:00.830-04:00</CREATION-TIME>
                diff from    : 2014-07-09T10:19:00-04:00
                diff to      : 2014-07-09T10:19:00.830-04:00

Reason: date format had sub second time offset in input, but generated value does not.

Implementations of ReqIF seem generally inconsistent with what ISO 8601 type of time is encoded.

We need to decide whether we enforce a particular ISO 8601 type time format (which means we update the specs), or detect the time format and re-generate the same when we output the round-tripped time (e.g. when we modify something and spit the file back out.