Consider extending `createInlinePauliStrSum` to accept `+` in-place of newline

[TysonRayJones]

This enables avoiding the clunky multiline string literal nuisance when the PauliStrSum contains only a few operations. Eg.

// C++
createInlinePauliStrSum(R"(
    0.123 XX
    1.23i YY
    -1-6i IZ
)");

// C
createInlinePauliStrSum(
    "0.123 XX   \n"
    "1.23i YY   \n"
    "-1-6i IZ   \n"
);

// agnostic
createInlinePauliStrSum("0.123 XX \n 1.23i YY \n -1-6i IZ");

can become the more palatable

createInlinePauliStrSum("0.123 XX + 1.23i YY -1-6i IZ");

Beware that this cannot be done by retaining our existing parsing/regex/validation and simply prior substituting encountered + for \n. We would fail to include \n before negative coefficients, and mutilate scientific notation with positive exponents.

[TysonRayJones]

For context, convenient syntax for these few-term PauliStrSum is motivated by the recent applyTrotterizedNoisyTimeEvolution() function which accepts Lindblad jump operators specified as PauliStrSum. And common jump operators include on- and off-diagonal projectors, which satisfy...

$$ \ket{0}\bra{0} = \frac{1}{2} I + \frac{1}{2} Z $$ $$ \ket{0}\bra{1} = \frac{1}{2} X + \frac{i}{2} Y $$ $$ \ket{1}\bra{0} = \frac{1}{2} X - \frac{i}{2} Y $$ $$ \ket{1}\bra{1} = \frac{1}{2} I - \frac{1}{2} Z $$

The need for this new syntax however remains terribly unimportant and probably outweighed by the nuisance of overhauling the string parsing ¯\_(ツ)_/¯