Fix power monitor buffer extraction

[Christian-B]

The data extraction from ChipPowerMonitorMachineVertex had been disabled due to a bug.

This PR and the associated ....

1. Add a new iterate_placements_by_vertex_type method
2. rename the previous one to iterate_placements_by_xy_and_type
   (Both on Placements and the View)
3. Use the new method where there either there is no Progressbar or where the ProgressBar did not make sense.
4. Merge buffer_extractor into BufferManager.
5. If using Java just use iterate_placements_by_vertex_type and avoid pre counting and storing
6. If using python count and store until Remove many progress bars PACMAN#468 decides to keep or remove
7. pre counter now uses iterate_placements_by_vertex_type as old version via AppVertex did not work as ChipPowerMonitorMachineVertex do not have app vertexes.

Must be done at the same time as:
SpiNNakerManchester/PACMAN#469

Tested by:
SpiNNakerManchester/IntegrationTests#142

[andrewgait]

Looks good to me.