Write design document for Device object

[thesamovar]

See title and issue #49.

[thesamovar]

I started writing this document on the wiki.

[mstimberg]

Regarding saving the arrays and the specifier system: I see two ways of implementing this:

1. Specifiers themselves are device dependent, e.g. to create a new array variable you would do something along the lines of device.create_array_variable('v', unit, len(self), ...)
2. Specifiers are device independent, but delegate array access to a device-dependent object, so creating a specifier would pass a device: ArrayVariable('v', unit, len(self), device, ...)

With the first option you have re-implement all specifiers for each device, but maybe we need the flexibility that comes with it? The classes are basically simple data container, anyway.

[thesamovar]

I slightly prefer option (2) but only very slightly so let's see what seems more convenient as we start implementing stuff?

[thesamovar]

Oh, and let's keep the discussion on issue #49.

[thesamovar]

OK I've updated the wiki page on devices, we should work from this now. It's not very detailed, but it has the core ideas. I think now we need to actually implement C++ standalone mode and then people can use that as a reference when implementing other modes.