PICA: Implement PICA Path

[pkiraly]

PICA path is a mean to address data elements within a record. Its definitions is available at https://format.gbv.de/query/picapath. A cataloge will be able to filter data elements and records based on this addressing (e.g. in #137). In the first round the following basic rules will be implemented:

path             ::=  ( tag | xtag ) occurrence? subfields?
tag              ::=  [012.] [0-9.] [0-9.] [A-Z@.]
xtag             ::=  "2" [0-9.] [0-9.] [A-Z@.] "x" number
occurrence       ::=  "/" occurenceValue
occurenceValue   ::=  number "-" number | occurrencePattern | "*"
occurencePattern ::=  [0-9] [0-9]? [0-9]?
subfields        ::=  [$.]? ( [A-Za-z0-9]+ | "*" )
number           ::=  [0-9]+