* src/HAB/Pica/Record/Field.php (hasSubfield): New function. Return true
  if field has subfield.

* 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.

* php/HAB/Pica/Record/TitleRecord.php (removeLocalRecord)
(addLocalRecord): Set and unset parent record reference.
* php/HAB/Pica/Record/LocalRecord.php (getTitleRecord)
(unsetTitleRecord, setTitleRecord): New functions. Set, get, unset
containing title record.
(addCopyRecord, removeCopyRecord): Set and unset parent record
reference.
* php/HAB/Pica/Record/CopyRecord.php (getLocalRecord)
(unsetLocalRecord, setLocalRecord): New functions. Set, get, unset
containing local record.

* php/HAB/Pica/Record/Record.php ($_parent): New variable. Containing
parent record.

* php/HAB/Pica/Record/LocalRecord.php (containsCopyRecord): New
function. Return true if local record contains a copy record.
* php/HAB/Pica/Record/TitleRecord.php (containsLocalRecord): New
function. Return true if title record contains a local record.

* php/HAB/Pica/Record/NestedRecord.php (containsRecord): New
function. Return true if nested records contains a record.
(addRecord): Use new function.

* NestedRecord.php (getFields): Do not delegate getFields() to
contained records when called with selector.

Record::select() calls getFields() w/o arguments which returns all
fields.

Returns the first of all fields that are matched by a field selector.

of fields identified by a tag