PeriodicGrid class with support for periodic boundaries

[tovrstra]

I took some time to explain in detail how it worked in the docstrings. It would be good to check if they are sufficiently accessible.

I've relied on pytest features to write exhaustive unit tests. They will not run without pytest.

I've also checked if this can be used to partition electron densities of periodic systems. That all seems to work.

[codecov]

Codecov Report

Merging #87 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #87   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     13    +1     
  Lines         805    901   +96     
=====================================
+ Hits          805    901   +96

Impacted Files	Coverage Δ
src/grid/periodicgrid.py	100% <100%> (ø)

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[FarnazH]

Thanks for the detailed docstrings and comments which made it clear and easier to understand.