Ensure long-term consistency and robustness in header usage

* Further separate source files under src/marsyas/core into
  - src/marsyas/system
  - src/marsyas/sched
  - src/marsyas/expr

* Merge src/marsyas/optional into src/marsyas/marsystems
  A separate dir for optional MarSystems did not bring any benefit,
  as individual files would still need to be included/excluded from
  compilation based on available dependencies.

* Fix all inclusions of public headers to the form: <marsyas/...>
  This brings consistency in how headers are referred to within the
  library and by third-party library users; it also allows a single
  include directory to be defined on command line.

* For the moment, do not install headers of individual MarSystems,
  until they are all cleaned up and good for public usage.