Unified Pair syntax for sublats kwarg in hopping

[pablosanjose]

Addresses one of the points in #19

This simplifies and extends the sublats keyword in hopping. A hoppings from sublat :A to sublat :B is now always denoted with :A => :B. The PR also allows for broadcasting between two groups of sublattices (:A, :B) .=> (:A, :B) (equivalent to (:A => :A, :B => :B)), and even direct product, (:A, :B) => (:C, :D) (equivalent to (:A=>:C,:A=>:D,:B=>:C,:B=>:D)).

Updated and extended also the docstrings for hopping and onsite, and added an internal method Quantica.sublats(model) that returns the sublats of each term in model.

[codecov-io]

Codecov Report

Merging #29 into master will increase coverage by 0.36%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##           master      #29      +/-   ##
==========================================
+ Coverage   47.93%   48.29%   +0.36%     
==========================================
  Files          14       14              
  Lines        2078     2081       +3     
==========================================
+ Hits          996     1005       +9     
+ Misses       1082     1076       -6     

Impacted Files	Coverage Δ
src/tools.jl	68.00% <77.77%> (+2.50%)	⬆️
src/hamiltonian.jl	42.41% <100.00%> (ø)
src/model.jl	70.34% <100.00%> (+1.56%)	⬆️

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