modifications to get maeparser building with C++20

[greglandrum]

As-is, maeparser will not compile with g++ using -std=gnu++20 because MaeBlock.hpp has some constructors and destructors which include explicit template information. I'm not sure this was ever required, but it's no longer allowed in C++20.
Here's a stack overflow post with some more details: https://stackoverflow.com/questions/63513984/can-class-template-constructors-have-a-redundant-template-parameter-list-in-c2

The fix is easy and shouldn't affect anything when using older versions of the C++ standard (I've tested back to C++11).

I found this because the RDKit had some of the same fun in it that needed to be cleaned up.

[ricrogz]

I merged master into the branch to trigger a build. But I didn't expect my push to go to your repo, @greglandrum. In fact, I didn't expect I'd be allowed to push to your repo!