Fix boundary construction in GS.Bands

[pablosanjose]

Closes #252

The issue was in the construction of orbsleft and orbsright LatticeSlices, which wrongly assumed that the sites coupled to a cell to the left (right) of the boundary spanned a single cell. These LatticeSlices are used to compute the G0 components that, combined, implement the boundary condition.

Note: the GS.Bands solver still allocates too much, specially with boundaries. There is probably still some low-hanging fruit here.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.31%. Comparing base (e35966a) to head (f2e3bde).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #253      +/-   ##
==========================================
+ Coverage   92.07%   92.31%   +0.23%     
==========================================
  Files          36       36              
  Lines        5857     5883      +26     
==========================================
+ Hits         5393     5431      +38     
+ Misses        464      452      -12     

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