potential problem with handling of build_options

[tnowotny]

Sorry, @mstimberg to add to load but I discovered the following when looking into issue #129.
Device build options are used e.g. to set use_GPU which determines whether GeNN shall build for GPU.
Normally, they are passed in set_device such as

set_device('genn', use_GPU= False)

The way it is put together at the moment, these options are caught through **kwargs and then passed around for a bit to eventually get saved in device.build_options here:
https://github.com/brian-team/brian2/blob/6e0374c628371aa4a04959df3fd423730f8e3337/brian2/devices/device.py#L338

However, if users call device.activate() directly, e.g. after resetting the device, and if they omit the build options, they are deleted. I got myself caught up in that and think it is potentially not very intuitive to know that the options passed to set_device() would have to also be passed when calling device.activate().

Should we refactor slightly so that device build options remain preserved when device.activate() is called (as they are when device.reinit() is called)?