Simplified, more robust default activities

From RTT 1.8 on, an Orocos component is created with a default 'SequentialActivity', which uses ('piggy-backs on') the calling thread to execute its asynchronous functions. It has been argued that this is not a safe default, because a component with a faulty asynchronous function can terminate the thread of a calling component, in case the 'caller' emits an asynchronous event (this is quite technical, you need to be on orocos-dev for a while to understand this).

Furthermore, in case you do want to assign a thread, you need to select a 'PeriodicActivity' or 'NonPeriodicActivity', which have their quirks as well. For example, PeriodicActivity serialises activities with equal period and periodicity, and NonPeriodicActivity says what it isn't instead of what it is.

The idea is to create a new activity type which allocates one thread, and which can be periodic or non-periodic. The other activity types remain (and/or are renamed) for specialist users that know what they want.