Convergence improvement for tp_flash

[prehner]

In some cases, when the different solutions found by a stability analysis are right above the equality threshold, an almost trivial solution is used to initialize the tp flash for cases that could otherwise be solved very robustly.

MRE:

from feos.si import *
from feos.pcsaft import *
from feos.eos import *
import numpy as np

pcsaft = PcSaftParameters.from_json(['cyclopentane', 'propylene'], '../input/esper2023.json')
eos = EquationOfState.pcsaft(pcsaft)

state = State(eos, 343.7964273175933*KELVIN, partial_density=np.array([78.77667866249813, 423.48238669680353])*MOL/METER**3)
state.tp_flash()