Accessing complex types via control server

This is from a question asked by Tom Willemsen.

If a device is defined like this:
```python
class MyDevice(StateMachineDevice):
def initialize_data(self):
self.channels = {1 : Channel(), 2 : Channel(), 3 : Channel()}
...
```

Where `Channel` is defined like this:
```python
class Channel(object):
def __init__(self):
self.waveform_type = 0
self.step_time = 0
...
```

Would it be possible to access via control server using this sort of syntax (or similar):
```
$ lewis-control device channels[1].waveform_type 1
```

For now I suggested creating `set_channel_param(self, index, param, value)` and `get_channel_param(self, index, param)` methods on the MyDevice object. But it would be nicer if we could avoid the need for creating such boilerplate methods.

There are two difficulties with implementing something like this:
- Syntax / shell interaction (`[1]` might mean something special in some shells)
- Trying to json-rpc `Channel` objects throws a "not serializable" exception

The method workaround works fine, so this issue is mainly to see if we can come up with ideas on how to improve this.