Dataframe/Field array syntax for filtering and indexing

[atbenmurray]

At the moment, dataframes have apply_filter and apply_index. In pandas , this is done through array dereference syntax. There isn't any particular reason why we can't also have the same array dereference syntax, and this would help people who are otherwise struggling to get their head around pandas style array dereference dataframe manipulations

[ericspod]

This means being able to use [] brackets with indices and slices and arrays of bools for selecting, eg. foo[[0,2,12]] or foo[1:30:4].

[Liyuan-Chen-1024]

To use [] brackets, means we need to rewrite __getitem__ function.
In fields.py, we already rewrited __getitem__ for different types of arrays.

So what this issue asked for, is to rewrite __getitem__ for fields.