Adding a small header to the benchmark utility

[Iximiel]

Description

I have been digging into many benchmarks lately, and this small addition eases the work for me and will be useful for any benchmark user in the future.
Now the first output of the benchmark will be something like this:

BENCH:  Welcome to PLUMED benchmark
BENCH:  Using --kernel=this
BENCH:  Using --plumed=plumed.dat
BENCH:  Using --nsteps=2000
BENCH:  Using --natoms=100000
BENCH:  Using --maxtime=-1
BENCH:  Using --sleep=0
BENCH:  Using --atom-distribution=line
BENCH:  Initializing the setup of the kernel(s)

That is for a run here is plumed benchmark with no options given, if an option like --shuffled is given a line with BENCH: Using --shuffled will appear.

I find this useful for reproducing the bench later, or to share a clearer file.
It may also be useful in the case of postprocessing benchmarks without needing to rely on inventing filenames that contain all the selected options.

Target release

I would like my code to appear in release 2.10

Type of contribution

• changes to code or doc authored by PLUMED developers, or additions of code in the core or within the default modules
• changes to a module not authored by you
• new module contribution or edit of a module authored by you

Copyright

• I agree to transfer the copyright of the code I have written to the PLUMED developers or to the author of the code I am modifying.

• the module I added or modified contains a COPYRIGHT file with the correct license information. Code should be released under an open source license. I also used the command cd src && ./header.sh mymodulename in order to make sure the headers of the module are correct.

Tests

• I added a new regtest or modified an existing regtest to validate my changes.
• I verified that all regtests are passed successfully on GitHub Actions.