Rename cellsites to sites and polish indexing API

[pablosanjose]

In the current unreleased master we may index x::Lattices and x::GreenSolutions using s = cellsites(cell, inds), like x[s] or x[s,s´], where x[s] === x[s,s].

This PR does three things:

• Renames cellsites to the simpler sites, which is already exported, and now takes on an additional new role. We intend this to be the stable API starting with Quantica v1.1.
• Allows one to use sites(inds) without cell, which defaults to a 0D cell::SVector{0,Int}. When indexing, we check if the cell is 0D, and in such case we sanitize it to be the zero cell of the appropriate lattice dimension. Hence cell-less indexing likelat[sites(inds)] behaves as zero-cell indexing lat[sites(Quantica.zerocell(lat), inds)].
• Implements indexing of h::AbstractHamiltonian, h[sites(...)] and h[sites(...), sites(...)]. This returns always a flat block, much like for GreenSolutions, but sparse.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.07843% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 92.52%. Comparing base (898a521) to head (89e40a6).
Report is 8 commits behind head on master.

Files	Patch %	Lines
src/sanitizers.jl	85.71%	1 Missing ⚠️
src/slices.jl	66.66%	1 Missing ⚠️

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #277   +/-   ##
=======================================
  Coverage   92.52%   92.52%           
=======================================
  Files          37       37           
  Lines        6101     6129   +28     
=======================================
+ Hits         5645     5671   +26     
- Misses        456      458    +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[pablosanjose]

Note: we are still missing indexing of AbstractHamiltonians with generic siteselectors. Since that is orthogonal to this PR, it should probably be done separately.