Refactor Gate, Operator definitions

[mofeing]

This PR refactors the Gate and Operator definitions. The difference gate types (e.g. X, Swap, Ry, CZ, ...) are still Operator subtypes, but now they are concrete subtypes that implement the parameters in their fields (unlike before, where parameters where encoded as a generic parameter of the Gate type).

This change lets us create operators without assigning the site they are acting on, which should be sufficient for retrieving the array representations or some other information. With this change, the Gate and Operator types have more clear semantics: A Gate is an Operator acting somewhere (i.e. has sites).

It also provides a small macro utility @gatedecl for declaring gate types.

Closes #44

[jofrevalles]

Suggested change

	`N` qubits. A new random `SU{N}` can be created with `rand(SU{N}, lanes...)`, where `N` is the dimension
	`log2(N)` qubits. A new random `SU{N}` can be created with `rand(SU{N}, lanes...)`, where `N` is the dimension

The definition of SU{N} (random unitary) is that N is the number of dimensions of the operator, therefore, in the case of qubits, this would be log2(N). This is how I have seen SU defined everywhere (see in Qiskit, in some papers, ...).

[mofeing]

Yep, but using N as the dimension of the matrix is super annoying. First, it doesn't tell you in how many sites it acts. Also I added a note to describe that N in our case is the number of sites, not the dimensionality.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 37 lines in your changes are missing coverage. Please review.

Project coverage is 57.58%. Comparing base (45abf2f) to head (88cfb0b).

Files	Patch %	Lines
src/Gate.jl	77.69%	29 Missing ⚠️
src/Array.jl	84.90%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
+ Coverage   56.02%   57.58%   +1.55%     
==========================================
  Files           5        5              
  Lines         282      323      +41     
==========================================
+ Hits          158      186      +28     
- Misses        124      137      +13     

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