Fixed Get-SevOneDevice

There were some issues around how it would handle not finding a device.
This function will now throw an error if it can't find a device based on
IP, ID, or name.
New-SevOneDeviceGroup

This works fairly well.  Just exposed a bug in the
AGet-SevOneDeviceGroup function.  It isn't failing when no objects are
returned.  Will work on this next.

Additionally, just as a general note, Device groups and Object Groups
should probably be handled by the same functions.