Affine scale factor is included in fitting parameters

[mccarthy677]

I built a signal processor that includes an affine shear and scale transformation. When I use make_LMFIT_params, the affine scale factor is included in the fit parameters, but this parameter is not one that should be fitted for. Can we change the default behavior to exclude this parameter from the list, or change it to not vary by default?

To Reproduce

Sample code:

from mrsimulator import Site, SpinSystem, Simulator
from mrsimulator import Method, SpectralDimension
from mrsimulator.method import SpectralEvent
from mrsimulator import signal_processor as sp
import numpy as np

mqmas = Method(
    channels=["87Rb"],
    magnetic_flux_density = 9.373412440496665, #in T
    rotor_frequency= np.inf, # in Hz
    spectral_dimensions= [
        SpectralDimension(
            count = 1079,
            spectral_width = 12500,
            reference_offset = -12931.260000005364,
            origin_offset = 131077732.42,
            label="3Q",
            events=[
                SpectralEvent(transition_queries = [{"ch1": {"P": [-3], "D":[0]}}])
            ]
        ),
        SpectralDimension(
            count = 1024,
            spectral_width = 25000,
            reference_offset = -4310.420000001788,
            origin_offset = 131077732.42,
            label="1Q",
            events=[
                SpectralEvent(transition_queries = [{"ch1":{"P":[-1], "D":[0]}}])
            ]
        )
    ],
)
site1 = Site(
    isotope="87Rb",
    isotropic_chemical_shift=-31,  # ppm
    quadrupolar={"Cq": 1.7e6, "eta": 0.58},  # Cq in Hz
)
sites = [site1]
RbNO3_spin_systems = [SpinSystem(sites=[s]) for s in sites]
sim = Simulator(spin_systems=RbNO3_spin_systems, methods=[mqmas])

processor = sp.SignalProcessor(operations=[
    # broadening and scaling
    sp.IFFT(dim_index=(0, 1)),
    sp.apodization.Gaussian(FWHM="0.05 kHz", dim_index=0),
    sp.apodization.Gaussian(FWHM="0.1 kHz", dim_index=1),
    sp.FFT(dim_index=(0, 1)),
    sp.Scale(factor=5e8),
    # first shear
    sp.IFFT(dim_index=1),
    sp.affine.Shear(factor='-21/27', dim_index=0, parallel=1), #applying to zero
    sp.IFFT(dim_index=0),  
    sp.affine.Scale(factor=48/27, dim_index=1),
    sp.FFT(dim_index=0),
    sp.FFT(dim_index=1),
    ]
)

from mrsimulator.utils import spectral_fitting as sf

fit_parameters = sf.make_LMFIT_params(sim, processor)
print(fit_parameters.pretty_print(columns=["value", "min", "max", "vary", "expr"]))

Desktop (please complete the following information):

• OS: Windows
• Python version: 3.9.18
• mrsimulator version: 0.8.0