Improve handling of GCMs in config file

[nhsavage]

At the moment, we have significant duplication of the models

  test:
    - meteo_france
    - ukmo
    - mme
  full:
    - meteo_france
    - dwd
    - cmcc
    - ncep
    - ukmo
    - ecmwf
    - jma
    - eccc
    - bom
    - mme

and:

services:
  ecmwf: [51, 1]
  meteo_france: [9, 1]
  dwd: [22, 1]
  cmcc: [35, 1]
  ncep: [2, 1]
  jma: [3, 0]
  eccc_can: [4, 1]
  eccc_gem5: [5, 1]
  ukmo: [604, 1]
  bom: [2, 1]
  mme: [1, 0]

Also since the refactoring for Python script it now fails on MME in test mode.

[nhsavage]

proposal. Just keep a single "centres" entry with optional weights and system numbers (to override values set in the code as defaults)

centres:
  - meteo_france
    - weight: 1.5
    - system: 52
  - dwd
  - cmcc
  - ncep
  - ukmo
  - ecmwf
  - jma
  - eccc
  - bom
  - mme

[nhsavage]

Need a well thought out warning if it fails. i.e. " Service no longer in use - see C3S www...........com for server status and model versions"