CoreNrn: support for rebalanced files.dat

[ferdonline]

For MMB (BBPP134-917) we need to be able to load a set of .dat files distributed in given way.
However for the moment CoreNeuron only reads it in plain RoundRobin, which is not suitable to load ranks a different number of files.

This PR

We introduce a mark value (-1 in this case) indicating no-data so that, even with round-robin, we can define precisely which rank will load a given data file. All ranks will be able to jump strides of size N_RANKS (at their respective offset) and find entries targeted at them. Upon seeing a -1 the rank knows it is done and can stop reading.

This is a minor addition which enabled us to load balance and run the MMB simulation

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 67.07%. Comparing base (2abbd25) to head (5a9d014).
Report is 70 commits behind head on master.

Files with missing lines	Patch %	Lines
src/coreneuron/io/nrn_setup.cpp	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3147      +/-   ##
==========================================
+ Coverage   67.06%   67.07%   +0.01%     
==========================================
  Files         571      569       -2     
  Lines      111319   111172     -147     
==========================================
- Hits        74658    74573      -85     
+ Misses      36661    36599      -62     

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[azure-pipelines]

✔️ d0f0cfe -> Azure artifacts URL

[azure-pipelines]

✔️ 005577e -> Azure artifacts URL

[pramodk]

@ferdonline : I am converting this to a draft assuming this is specific to our custom manipulation of files.dat for MMB simulation.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[azure-pipelines]

✔️ 5a9d014 -> Azure artifacts URL

[bbpbuildbot]

Logfiles from GitLab pipeline #238749 (:white_check_mark:) have been uploaded here!

Status and direct links:

• ✅ mac_m1_cmake_build: [cmake, ON, OFF, OFF, address]
• ✅ spack_setup
• ✅ build:nmodl
• ✅ build:neuron:nmodl:intel:legacy
• ✅ build:neuron:nmodl:intel:shared
• ✅ build:neuron:nmodl:nvhpc:acc:legacy
• ✅ build:neuron:nmodl:nvhpc:acc:shared
• ✅ build:neuron:nmodl:nvhpc:omp:legacy
• ✅ build:neuron:nmodl:nvhpc:omp
• ✅ test:neuron:nmodl:intel:legacy
• ✅ test:neuron:nmodl:intel:shared
• ✅ test:neuron:nmodl:nvhpc:acc:legacy
• ✅ test:neuron:nmodl:nvhpc:acc:shared
• ✅ test:neuron:nmodl:nvhpc:omp:legacy
• ✅ test:neuron:nmodl:nvhpc:omp

[ferdonline]

@pramodk PR updated with a more informative comment

[pramodk]

Once Michael confirms above, we can merge this.