Replace macro usage for constants

[Lestropie]

Bears some relation to #2111, in that it might be nice to address old code structure as part of a general cleanup potentially in #2585.

Preprocessor macros are regularly used to define default values for parameters, often in cases where this value may be controllable at the command-line and it is desired for the default value to be printed as part of the help string for that option. However these are not properly context-bound, and in some instances I think I've found duplicate macros purporting to serve the same purpose but used in different places.

These days I try to use eg. a constexpr in the relevant namespace; eg. 7ae2f62. But it could be worthwhile to do an audit and clean up all residual lazy macro usage.

[daljit46]

I agree that #define statements need to go, and constexpr probably can replace the vast majority of use cases. However, I'm not sure whether the c++17 branch is the right place for this change (as I believe this is tangential). More importantly, I think we should be careful at amassing a large amount of changes in one pull request because it will most likely cause subtle changes in behaviour and bugs that may be hard to detect and non-obvious at first sight.

[Lestropie]

It might be the case that this and #2111 fit together as sort of "substitution of old code practises out of the back end". If there are some aspects of #2585 that end up being left out of the main C++17 transition due to not being strictly necessary, eg. something like adopting std::byte, that might then appropriately fall into this new category. But agree it is technically a different focus to "adoption of C++17".