spice_ddg_methods: added DDG.getOrdinal() documentation
spice_ddg_methods.md: added documentation for DDG.getDateFromString
spice_ddg_methods.md: documented DDG.isRelevant
spice_ddg_methods: reworded and reformatted documentation for a few functions; changed DDG.getDateFromString() to specify that input strings must be ISO 8601 format
spice_ddg_methods: renamed parameter in DDG.getDateFromString to "date" (previously was "date_as_string")
Merge pull request #29 from moonlightdrive/master

spice_ddg_methods: additional documentation of DDG functions