sc class library: TempoClock: allow setting time base at creation
sclang: allow changing TempoClock's beats
sclang: TempoClock: use logical time instead of physical time

* Initialize base seconds to current thread's logical seconds.

* When scheduling from another thread, do it relative to the thread's
  logical seconds translated to clock's beats.
documentation: update info on TempoClock (new use of logical time)