added Scatter and Gather implementations in PLUMED Communicator

[aalhossary]

Description

The MPI Communicator does not have Scatter and Gather implementations (It has AllScatter but not Scatter).
Here is an implementation of them for future use.
I need them in my development.

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.

[GiovanniBussi]

thanks for your contribution! Can you also add regtets? I guess the correct places are:

• regtest/basic/rt-make-2, to test the behavior when compiled with MPI
• regtest/basic/rt-make-7, to test the behavior when compiled without MPI

It should be as simple as:

• edit the main.cpp file
• type "make"

Note that test number 7 uses asserts, whereas test number 2 writes on a file. The second approach is a bit better, because in case of multiple functions it allows to see that only one failed. The first approach will just crash. Please follow the convention in the respective file.

Thanks again!

[Iximiel]

Please make more clear who is sending the data and break some line, like this:

Suggested change

	for(unsigned i=0;i<gather_send.size();i++) gather_send[i]=(i+1)*(comm.Get_rank()+1);
	for(unsigned i=0;i<gather_send.size();i++) {
	gather_send[i]=1+i+(comm.Get_rank()+1)*100;
	}

So we expect 101 102 103 104 201 202 203 204 301 302 303 304, and if something is wrong is faster to understand where

If you want you can apply a similar idea also to the scatter test