Generate a function that accepts an arbitrarily deeply nested list.
Use flatten.at to set the level of nestedness at which the flatterner
will start to flatten.
Usage
make_flattener(flatten.at = 1)
Arguments
flatten.at
An integer specifying the layer after which to
start the flattening. 1 means to start at the very top.
Value
A function that can be used to flatten a nested list