Supress warnings in MPValues (Fix #244)

[codecov-io]

Current coverage is 93.41% (diff: 100%)

Merging #245 into master will decrease coverage by 0.22%

@@             master       #245   diff @@
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  Files            72         72          
  Lines          5053       4999    -54   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits           4732       4670    -62   
- Misses          321        329     +8   
  Partials          0          0          

Diff Coverage	File Path
•••••••••• 100%	cpp/source/deal.II/mp_values.templates.h

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[Rombur]

OK ready for review