Replace loop with explicit formula

[MadCatX]

Iterative calculation of the number of bins in Parallel2DExecutor can be replaced by an explicit formula.

Test code:

#include <cmath>
#include <iostream>

int main()
{
        for (int nCpus = 2; nCpus < 513; nCpus++) {
                int levels = 1;
                while (1 << levels < nCpus)
                        levels++;
                levels++;
                int bins = 1 << levels;

                int levelsNew = std::ceil(std::log2(nCpus)) + 1;
                int binsNew = 1 << levelsNew;
                if (bins != binsNew)
                        throw std::runtime_error(std::string("MISMATCH ") + std::to_string(bins) + " " + std::to_string(binsNew));
                std::cout << nCpus << "(" << levels << ", " << bins << ") - " << "(" << levelsNew << ", " << binsNew << ")" << std::endl;
        }
}

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This change is 

[codecov-commenter]

Codecov Report

Merging #705 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #705      +/-   ##
==========================================
- Coverage   49.35%   49.35%   -0.01%     
==========================================
  Files         556      556              
  Lines       71298    71295       -3     
==========================================
- Hits        35188    35186       -2     
+ Misses      36110    36109       -1     

Impacted Files	Coverage Δ
SimTKcommon/src/Parallel2DExecutor.cpp	88.07% <100.00%> (+0.57%)	⬆️

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