Added sigterm to the signal handler for Benchmark

[Iximiel]

Description

I added the handler for SIGTERM to the benchmark CL.
I tried with kill -TERM pid and it seems to work
I made the info message more pedantic.

@GiovanniBussi I added the handler correctly or should I add another function?

Target release

I would like my code to appear in release XXXXX

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.

[Iximiel]

The motivation is that on an HPC queue the signal sent on "time expired" usually is SIGTERM and as now the benchmark exit gracefully only with SIGINT