#1099: Count phases as unsigned and avoid mixed arithmetic/comparisons

[PhilMiller]

Fixes #1099

[lifflander]

I do prefer uint32_t, but this is fine.

[PhilMiller]

And I prefer languages that implement faithful math, and don't make programmers dance around conversion rules :-)

[codecov]

Codecov Report

Merging #1101 into develop will not change coverage.
The diff coverage is 50.00%.

@@           Coverage Diff            @@
##           develop    #1101   +/-   ##
========================================
  Coverage    77.71%   77.71%           
========================================
  Files          671      671           
  Lines        25855    25855           
========================================
  Hits         20092    20092           
  Misses        5763     5763           

Impacted Files	Coverage Δ
...c/vt/vrt/collection/balance/model/composed_model.h	100.00% <ø> (ø)
src/vt/vrt/collection/balance/model/linear_model.h	100.00% <ø> (ø)
src/vt/vrt/collection/balance/model/load_model.h	100.00% <ø> (ø)
...t/vrt/collection/balance/model/naive_persistence.h	100.00% <ø> (ø)
.../vt/vrt/collection/balance/model/per_collection.cc	76.92% <0.00%> (ø)
...c/vt/vrt/collection/balance/model/per_collection.h	100.00% <ø> (ø)
...llection/balance/model/persistence_median_last_n.h	100.00% <ø> (ø)
src/vt/vrt/collection/balance/model/raw_data.h	100.00% <ø> (ø)
src/vt/vrt/collection/balance/node_stats.h	100.00% <ø> (ø)
...nit/collection/test_model_multiple_phases.nompi.cc	86.66% <0.00%> (ø)
... and 12 more

[lifflander]

Looks good