added checks for empty BIC in CdtrAgt
[TASK] Merged optional BIC and improved test
Different scenarios for the validation are still missing
adjustable format for creation date time

A local time format with utc offset ("'Y-m-d\TH:i:s.000P")
is required by a german bank (sparkasse-koelnbonn.de).
Added code documentation for the unique message identification parameter
Fix pain.008.001.02 for Spanish banks

Fix header for Spanish banks, see https://github.com/digitick/php-sepa-xml/issues/44
Shema,xml and Test for pain.008.003.02 added
BIC now only Optional
added test for single transaction without BIC
[TASK] Renamed test to match convention