Factor out Hamiltonian builders

[pablosanjose]

This is a first step towards addressing #4

Here we split (internally) the process of building a Hamiltonian, which was formerly a single step hamiltonian(lattice, model; orbitals), to a multistep process: b = builder(lattice; orbitals) (of type ParametricHamiltonianBuilder), add!(b, model), and finally hamiltonian(b). We can also do push!(b, modifiers...) to inject Modifiers into the builder before assembly into a Hamiltonian.

These builder and add! functions are unexported for the moment, and meant to be used internally only. add! also has a method add!(b, value, ::CellSites, ::CellSites) to add a single or a group of values to a Hamiltonian build before assembly. This is not exported for a good reason: we don't do matrix-size checks of value for performance reasons here. Hence, improper use in inhomogeneous multiorbital cases could in principle lead to malformed block elements (i.e. with nonzeros out of place).

The advantage of this refactor is that we can now envision making a Wannierization object that wraps two builders that are populated as the wannier90 file is read, one for the Hamiltonian and another for the position matrix elements. Then, it becomes possible to combine the hamiltonian builder inside this object with other models (even parametric models) so that we can mix Wannier90 presents with flexible parametric models and modifiers.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.58170% with 19 lines in your changes are missing coverage. Please review.

Project coverage is 92.30%. Comparing base (a26d181) to head (a83b850).
Report is 1 commits behind head on master.

Files	Patch %	Lines
src/builders.jl	83.11%	13 Missing ⚠️
src/hamiltonian.jl	92.18%	5 Missing ⚠️
src/specialmatrices.jl	50.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #261      +/-   ##
==========================================
- Coverage   92.48%   92.30%   -0.19%     
==========================================
  Files          36       36              
  Lines        5885     5942      +57     
==========================================
+ Hits         5443     5485      +42     
- Misses        442      457      +15     

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