Add selector_hartree and selector_fock to meanfield

[pablosanjose]

When doing meanfield calculations, sometimes we want long-range Hartree interactions (which take the form of purely onsite terms), but don't need long-range Fock terms (which induce long-range hoppings, which in turn often make calculations more expensive).

Currently, the selector kwarg in meanfield sets the interacting site pairs for building both the Hartree onsite terms and the Fock hopping terms. The only way to switch off Fock is to set fock = 0, but that doesn't eliminate the Fock hopping terms, it just makes them zero, producing unnecessary overhead in some solvers (e.g. GS.Schur, which then requires building a large supercell to preserve only nearest-cell hoppings).

In this PR we provide selector_hartree that applies to the calculation of Hartree onsite terms, overriding the general selector that applies to both Hatree and Fock by default. In this way, we can set selector = (; range = 0) (i.e. the default), fock = 0 and selector_hartree = (; range = large_distance), and have long-range Hartree-only meanfields without the long-range hopping overhead.

EDIT: we also add selector_fock for completeness and symmetry. Both default to selector, which is the one to use when we don't want to make a difference, just as potential is the one to use, instead of hartree and fock, when we want to treat hartree and fock consistently.

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.45%. Comparing base (8a8b193) to head (8865823).

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Additional details and impacted files

@@           Coverage Diff           @@
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  Coverage   92.45%   92.45%           
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  Files          41       41           
  Lines        7074     7074           
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  Hits         6540     6540           
  Misses        534      534           

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