Add more fixes to modify PICA+ records

[nichtich]

Here path denotes a PICA Path (such as 003@$0), field a field reference (such as dc.creator), value a string value and contents a field content (e.g. $afoo$bbar).

• pica_update(path, values|contents, [options]) replace or add subfields or fields (deprecates pica_add and pica_set)
• pica_replace(path, pattern, value) to search and replace in PICA subfield values with regular expression
• pica_subfields(code,value,code,value...) or pica_subfields(contents) to set subfields of current field(s)
• pica_annotate([path,] annotation) to set annotation
• pica_append([path,] value|contents) to add full field(s) or subfield(s) (maybe not needed)
• pica_sort([path,] schedule) to sort and reduce to known (sub)fields
• pica_reduce([path,] schedule) to reduce to known (sub)fields (?)

Existing fixes:

• pica_add(field, path, [options]) : should be depreacted because of order of arguments
• pica_set(field, path, [options]) : might be depreacted because of order of arguments. Alternative name?
• pica_keep(path [, path...])
• pica_remove([path])
• pica_map(path, field)
• pica_tag(tag)
• pica_occurrence(occurrence)