Instrument channels

Channels need "nicer" syntax and behaviour:
Syntax:

``` python
instruments.channel_list[0].volt
voltages = [0,1,2,1,0]
instrument.channel_list.volt(voltages)
```

@qdev-dk/qcodes anybody explain and/or expand the following syntax? Copied from #265.

``` python
instruments.channel_list[0].set_async(0) # setting channel 0 async, or not async ?
instruments.channel_list[1].set_async(1) # setting channel 1 async, or not async ?
# how to set all of them ?
instruments.channel_list.do_things()
```

The issue names no files, tests, or entry points; begin by locating the instrument channel-list implementation and reviewing the related discussion in issue #265. Clarify the intended channel access, bulk operations, and asynchronous-setting behavior before implementation, then define tests for the accepted syntax and behavior.

Written by the indexing model from the issue text.