PDO tag should be customizable for various samples

[prakashngit]

PDO tag used for various contracts should be customizable, rather than forcing all examples within this folder to use the same PDO version.

Best to export the PDO version as part of the installation of the contract, and check whether the required PDO version is installed. Also, perhaps no need to carry PDO as a submodule of the contracts folder itself.

[cmickeyb]

I don't know why we would want to support multiple versions of the PDO source for different contract families. It seems like that would 1) make automated testing impossibly complex 2) make it extremely difficult for anyone who wants to use the contracts, 3) be inconsistent with any attempt at broad deployment.

Personally, I think a much better approach is to require all contract families to support the same version of PDO. This means that anyone who contributes a contract family will be responsible to continue to provide updates. When new versions of PDO come out, we should make a branch with the new version until all contract families pass their own automated tests.