implement arrays

[gschnabel]

I've implemented simple array functionality and associated tests. Now syntax like x[3] or x[2][4] can be used for accessing elements in simple arrays and nested arrays, respectively.

Importantly, the underlying Python data type should be list so that it works as expected. Therefore, in contrast to numpy arrays, the shape of arrays can be irregular (e.g., different number of elements per row). I think this is a good choice if the central data structure we want to manipulate/transform is data in the JSON format, for which the definition of arrays mirrors the Python list data type.

[thomasms]

This looks great. I just had a few small comments.

I also tried _parse_and_assert("[1,4,1,1,-9]", [1,4,1,1,-9]) as a test but I guess this only does array evaluation, and does not support hard coded arrays.