Give PauliEvolutionGates the same name-attribute for all instances

[Cryoris]

Summary

Update the PauliEvolutionGate such that the name attribute is always "PauliEvolution". The information on how the gate should be drawn is moved to label (unless the user overrides the label).

Details and comments

This allows unrolling a circuit to PauliEvolutionGates as

unrolled = Unroller(["PauliEvolution", "u", "cx"])(circuit)

Also, this PR includes a small update to how Pauli evolutions are drawn by adding brackets around sum of operators. For example, before the evolution of ZZ + IX was drawn as

             ┌───────────────────────┐
        q_0: ┤0                      ├
             │  exp(-i ZZ + XI)(0.2) │
        q_1: ┤1                      ├
             └───────────────────────┘

and now contains brackets (and -i is changed to -it to indicate a time parameter)

             ┌──────────────────────────┐
        q_0: ┤0                         ├
             │  exp(-it (ZZ + XI))(0.2) │
        q_1: ┤1                         ├
             └──────────────────────────┘

[coveralls]

Pull Request Test Coverage Report for Build 2007465703

• 10 of 10 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 83.506%

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Totals
Change from base Build 2007464696:	0%
Covered Lines:	52472
Relevant Lines:	62836

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💛 - Coveralls