foreach_cell iteration should continue until at least one site is found

[pablosanjose]

In supercell searches without a specific list of cells, we ensure BoxIterator keeps going until we find at least one site (or we exceed the maximum iterations). However foreach_cell did not have this check, and would stop as soon as a closed shell without hits was formed. This led to lattice slicing (using a region such as a ring around the origing) where we could fail to form a slice. Note also that siteselector has no seed option currently (unlike supercell), so this problem could not be worked around.

Perhaps a future refactor could employ foreach_cell and foreach_site in supercell, to be more DRY.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (db3b60a) 92.58% compared to head (743016a) 92.59%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #222   +/-   ##
=======================================
  Coverage   92.58%   92.59%           
=======================================
  Files          34       34           
  Lines        5450     5457    +7     
=======================================
+ Hits         5046     5053    +7     
  Misses        404      404           

Files	Coverage Δ
src/apply.jl	92.51% <100.00%> (+0.16%)	⬆️
src/selectors.jl	98.80% <100.00%> (+0.04%)	⬆️

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