Changed mutables to std::atomic in PixelCPEBase

To avoid complaints from the static analyzer, converted mutable
debug counters to std::atomic. These debug counters are only used
if a CPP variable is set. This change compiles when that variable is
set, however other code in this package fails even without this change.
Merge pull request #3701 from Dr15Jones/mutableInPixelCPEBase

Reco -- Changed mutables to std::atomic in PixelCPEBase