calculate system cores before getMachine can be called

[Christian-B]

These PRs calculate the resources needed for monitor cores before a run is stated and a machine in requested.

This has 2 functions.

1. The ChipCounter is more accurate and will work for Spin2

2. Allow users to request n_cores in setup
   To convert that to n Chips and N boards we need to know how many cores per chips will be taken by the monitor cores.

Therefor we now support the idea of adding Sample (never to be placed) monitor vertices.

We track the monitor vertices so can calculate sdram when we have all the required data

As the SDRAM may depend on the runtime the calculation of the SDRAM cost of these vertices is delayed until needed.
Only core count is needed for get_machine
ChipCounter is only used during simulator run.

Must be done together with:
SpiNNakerManchester/SpiNNFrontEndCommon#1164

allows:
SpiNNakerManchester/MarkovChainMonteCarlo#63

tested by http://apollo.cs.man.ac.uk:8080/blue/organizations/jenkins/Integration%20Tests/detail/monitor_size/1/pipeline/

[coveralls]

coverage: 86.186% (+0.04%) from 86.142%
when pulling 912177b on monitor_size
into f689c54 on master.