Allow for Pauli strings that have only operators

In #984, @jlbosse has improved our functionality for creating PauliTerm and PauliSum objects from their string representations. However, there were some underlying issues with the pyquil.paulis module before he started. He captured a collection of strings that do not work as part of the unit tests he added. One of them is the following, a pauli string which is just the operator without a coefficient. I think the behavior should be changed so that this is a valid string, and the coefficient should be assumed to be 1.

In [2]: PauliTerm.from_compact_str("X0")
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
~/code/rigetti/github/pyquil/pyquil/paulis.py in from_compact_str(cls, str_pauli_term)
    362             str_coef, str_op = re.split(r'\*(?![^(]*\))', str_pauli_term,
--> 363                                         maxsplit=1)
    364         except ValueError:

ValueError: not enough values to unpack (expected 2, got 1)

During handling of the above exception, another exception occurred:

ValueError                                Traceback (most recent call last)
<ipython-input-2-05e743997d63> in <module>
----> 1 PauliTerm.from_compact_str("X0")

~/code/rigetti/github/pyquil/pyquil/paulis.py in from_compact_str(cls, str_pauli_term)
    363                                         maxsplit=1)
    364         except ValueError:
--> 365             raise ValueError("Could not separate the pauli string into "
    366                              f"coefficient and operator. {str_pauli_term} does"
    367                              " not match <coefficient>*<operator>")

ValueError: Could not separate the pauli string into coefficient and operator. X0 does not match <coefficient>*<operator>

Inspect pyquil/paulis.py at PauliTerm.from_compact_str and review the unit tests added in #984 for unsupported string representations. Verify that an operator-only string such as "X0" is accepted with an assumed coefficient of 1, and run the relevant Pauli parsing tests.