Improve TangoSchemeTest device server
Add two new attributes, "float_scalar_poll" and "float_scalar_evt", the first one has been configured to emit Period events (the tango polling has been enabled), and the second one has been configured to emit Change events.
There are two new commands to configure the periods of these events: "SetEventPeriod" to set fixed change events, and "ChangePollingPeriod" to change the polling period of "float_scalar_poll" attribute.