re-add some statistics

[ArneBinder]

This reverts some of the changes of #366, i.e. it re-adds the following statistics (and simplifies their tests):

• TokenCountCollector: Collects the token count of a field when tokenizing its content with a Huggingface tokenizer.
• FieldLengthCollector: Collects the length of a field, e.g. to collect the number the characters in the input text.
• SubFieldLengthCollector: Collects the length of a subfield in a field, e.g. to collect the number of arguments of N-ary relations.
• DummyCollector: A dummy collector that always returns 1, e.g. to count the number of documents.

Note, that the SpanLengthCollector stays in pie-datasets, because it uses pie_datasets.document.conversion.tokenize_document.