New 3 si pixel2 d template db object - targeting 73X

The AlCaDB meeting has approved, targeting 73X
Automatically ported from CMSSW_7_2_X #5218
@hidaspal 
I agree with the comment by @VinInn . What I can suggest is:

1. try to figure out the maximum size of the vector with the current use-cases
2. use `shrink_to_fit`
3. try to find a dynamic way to reserve space (e.g., when building the object, if you know the size in advance, you can pass this value in the c'tor).