`ConfigSet` from arbitrary iterable & subscriptable `ConfigSet`

[gelzinyte]

Moving discussion from PR #222 here. (link to the thread)

First, have an __gettitem__ method

from itertools import islice
.
.
.
  def __getitem__(self, arg):
      if isinstance(arg, slice):
          return islice(self, arg.start, arg.stop, arg.step)
      elif isinstance(arg, int):
          return list(islice(self, arg, arg + 1, 1))[0]
      else:
          raise TypeError

Second, make it so ConfigSet can be constructed from an arbitrary iterable.

This came from wanting a (slightly) simpler interface, where ConfigSet behaves more like a list:

subsample_configset = ConfigSet(in_configset[::2])

Currently that can be done by converting to list and back

subsample_configset = ConfigSet(list(in_configset)[::2])

Not sure if this simpler interface is worth the changes, but being able to create ConfigSets from arbitrary iterable/generator might be useful?

[bernstei]

For this to be a sensible addition we really have to think carefully about what this means. When you create the new ConfigSet, should it be in memory? Or should it just keep a reference to the input iterable and step through it ass needed? What about an iterable that contains other iterables, perhaps lists, perhaps arbitrary?