Allow colocating multiple sequential PyFunc elements into a single Gst-plugin

[bwsw]

Currently, every function is colocated in a separate Gst plugin, which is inconvenient when multiple elements go one after another.

The feature allows the creation of a special unit: pymultifunc, which allows colocating multiple functions.

The colocated functions must be wrapped in individual telemetry spans to enable their profiling.

    - element: pymultifunc
      name: somename
      dev_mode: true/false
      elements:
        - module: samples.traffic_meter.mod1
          class_name: Op1
          kwargs:
            ...
        - module: samples.traffic_meter.mod2
          class_name: Op2
          kwargs:
            ...