Add missing factor of pi to parameterized Quil gate output

[karalekas]

This PR addresses a bug in Quil output.

The current code doesn't distinguish between Cirq's PowGate exponents and the angle parameters for Quil gates, but they differ by a factor of pi. This is verified by comparing program unitaries before and after conversion to Quil (see below).

Adds a test for this change that depends on pyQuil, so pyquil is added to pip-list-dev-tools.txt, but in the event that pyQuil is not available for import, the test is skipped.

Additionally sneaks in a change to Quil output for ISwapPowGate -- uses the native XY Quil gate when the exponent of the ISwapPowGate does not equal 1.

In [1]: import numpy as np
   ...: from pyquil import Program
   ...: from pyquil.simulation.tools import program_unitary
   ...:
   ...: from cirq import Circuit, LineQubit, QuilOutput, rx
   ...:
   ...: q0 = LineQubit(0)
   ...:
   ...: # rx(pi/2) creates an XPowGate with exponent 0.5 (and global_shift -0.5) 
   ...: circ = Circuit(rx(np.pi/2)(q0))
   ...: print(circ)
   ...:
   ...: pyquil_program = Program(str(QuilOutput(circ, (q0,))))
   ...: print(pyquil_program)
   ...:
   ...: pyquil_unitary = program_unitary(pyquil_program, n_qubits=1)
   ...: print(pyquil_unitary)
   ...:
   ...: cirq_unitary = circ.unitary()
   ...: print(cirq_unitary)
   ...:
   ...: np.allclose(pyquil_unitary, cirq_unitary)

on master:

0: ───Rx(0.5π)───
RX(0.5) 0

[[0.96891242+0.j         0.        -0.24740396j]
 [0.        -0.24740396j 0.96891242+0.j        ]]
[[0.70710678+0.j         0.        -0.70710678j]
 [0.        -0.70710678j 0.70710678+0.j        ]]
Out[1]: False

this PR:

0: ───Rx(0.5π)───
RX(pi/2) 0

[[0.70710678+0.j         0.        -0.70710678j]
 [0.        -0.70710678j 0.70710678+0.j        ]]
[[0.70710678+0.j         0.        -0.70710678j]
 [0.        -0.70710678j 0.70710678+0.j        ]]
Out[1]: True

Related to #1742, #2386, #2983

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[karalekas]

@googlebot I signed it!

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[balopat]

Thank you for all the fixes @karalekas, this looks good to me.

[vtomole]

LGTM