Add inelastic sample component that changes neutron wavelengths

[nvaytet]

This PR adds a new type of component: InelasticSample.
It modifies the neutron energies according to a probability function for energy shift.

It is used as follows:

# Sample 1: Uniform probability distribution
sample1 = tof.InelasticSample(
    distance=28.0 * meter,
    name="sample",
    delta_e=sc.DataArray(
        data=sc.ones(sizes={'e': 100}),
        coords={'e': sc.linspace('e', -0.2, 0.2, 100, unit='meV')},
    ),
)

# Sample 2: normal distribution
x = sc.linspace('e', -0.2, 0.2, 100, unit='meV')
sig = sc.scalar(0.03, unit='meV')
y = 1.0 / (np.sqrt(2.0 * np.pi) * sig) * sc.exp(-((x / sig) ** 2) / 2)
y.unit = ""

sample2 = tof.InelasticSample(
    distance=28.0 * meter,
    name="sample",
    delta_e=sc.DataArray(data=y, coords={'e': x}),
)

tof.Model(source=source, components=choppers + detectors + [sample1])

Internal changes

Quite a bit of the internals had to be refactored, because of the way the chopper and detector logic was hard-coded inside the Model (especially the run function).
Instead of taking in choppers and detectors, the Model now accepts a flat list of components.
The effects of each component are applied onto the neutrons in succession, and the logic for those effects has been moved inside the components themselves.

I think it makes for a much cleaner structure in the model. Logic for plotting was also moved into the respective components.

Fixes #117

[nvaytet]

@bingli621 please take a look (and also try it out!)

[nvaytet]

This is where we need to also update the dashboard to support samples. This will be done in a follow up PR.

[nvaytet]

Comments from in-person discussion:

• Use a callable instead of a data array with probability distribution to apply the sample effects on the neutrons (more flexible)
• Prevent neutrons from becoming unphysical (large DeltaE for large wavelengths could mean the final energy is less than zero)