Wrapper FMT header for compatibility

[v4hn]

for special eigen formatter, which is not available in fmt9+ (found on Debian testing).

fmtlib/fmt#3465
https://stackoverflow.com/a/73755864/21260084

@rhaschke Not sure whether you are ok with the generic wrapper header as you usually argue for minimal includes. The concrete issue currently only affects a single location where an Eigen datatype is formatted. But I believe as we use libfmt all over the place (as you introduced at some point), I would also expect to be able to format Eigen datastructures everywhere without additional includes & helpers.

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[rhaschke]

In principle I approve. But the name of the wrapper include (fmt_p.h) is too cryptic.
What is the meaning of _p?
I suggest simply fmt.h or fmt_wrapper.h.

[v4hn]

But the name of the wrapper include (fmt_p.h) is too cryptic.
What is the meaning of _p? I suggest simply fmt.h or fmt_wrapper.h.

You introduced that suffix and it implies that the header will not be installed.
I did not want to provide such a generic wrapper with MTC in general - also to avoid potential ODR violations -, but still provide it for all the subfolders in the core package.

[rhaschke]

You introduced that suffix and it implies that the header will not be installed.

I see. There the _p suffix stands for private. I'm okish to use that suffix here as well - now that I know ;-)
But, I would be ok as well to install the header. This doesn't immediately trigger ODR violations.

[v4hn]

now that I know ;-)

🃏

But, I would be ok as well to install the header. This doesn't immediately trigger ODR violations.

I'm aware. I prefer not to install it unless/until we need it in some very related downstream package too.

[rhaschke]

It is easily possible that we will use fmt in another package of this repo (aside from core) in the future.

[v4hn]

until we need it

It's not an external API for now and it takes one sed to change it if needed. 🐕