Implement shape detection in internal WithCmd tool

[GiovanniBussi]

In #1050 I implemented detection of the shape of arrays in the external wrapper class (Plumed::cmd). This is available for MD codes linking plumed. When using the cmd functions internally (e.g. in driver) it is still necessary to provide explicitly the shape or renounce to shape checking.

I would like to implement the same logic that is now in wrapper/Plumed.h also in core/WithCmd.h. It might be slighlty simpler since we can use c++17 there (and if constexpr).