* src/HAB/Pica/Record/Subfield.php ($validSubfieldCodePattern): New property.
  (isValidSubfieldCode): Factor out subfield code pattern.

* src/HAB/Pica/Record/TitleRecord.php (setFields): Properly handle
  record w/o local record fields.

* HAB/Pica/Record/Field.php (getSubfieldsWithCode): New
  function. Return all subfields with specified code.

Also includes cosmetic fixes wrt coding-style.