more concise array and string formatting

[orm011]

make array columns print only a few elements by using numpys array formatting functions also: make string columns print an abridged string if the original string is too long

rendered rag notebook here: https://github.com/orm011/pixeltable/blob/concise_format/docs/release/tutorials/rag-operations.ipynb

compare with: https://github.com/pixeltable/pixeltable/blob/master/docs/release/tutorials/rag-operations.ipynb