Convert `sc` to `cell`

[zerothi]

This fixes #95

[codecov[bot]]

Codecov Report

Merging #99 into master will decrease coverage by 0.01%. The diff coverage is 84.65%.

@@            Coverage Diff             @@
##           master      #99      +/-   ##
==========================================
- Coverage   84.69%   84.67%   -0.02%     
==========================================
  Files         112      112              
  Lines       17493    17526      +33     
==========================================
+ Hits        14815    14840      +25     
- Misses       2678     2686       +8

Impacted Files	Coverage Δ
sisl/io/molden.py	100% <ø> (ø)	:arrow_up:
sisl/io/siesta/orb_indx.py	96.96% <ø> (ø)	:arrow_up:
sisl/io/tbtrans/_cdf.py	91.01% <100%> (ø)	:arrow_up:
sisl/io/tbtrans/delta.py	94.69% <100%> (ø)	:arrow_up:
sisl/io/xyz.py	87.27% <100%> (ø)	:arrow_up:
sisl/_array.py	100% <100%> (ø)	:arrow_up:
sisl/io/siesta/kp.py	100% <100%> (ø)	:arrow_up:
sisl/__init__.py	100% <100%> (ø)	:arrow_up:
sisl/geom/basic.py	96.42% <100%> (ø)	:arrow_up:
sisl/conftest.py	96.59% <100%> (ø)	:arrow_up:
... and 30 more

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[zerothi]

This will be superseeded by a change of sc to lattice, as per discussion.