Serialization of Simulator.config.decompose_spectrum

[mgiammar]

Steps to reproduce:

1. Create a new file in the mrsimulator-app
2. Add two spin_systems to the file
3. Add a method to the file
4. Under Signal Processing add two convolutions
5. Have the first convolution apply only to spin_system 0 and second convolution only apply to spin_system 1
6. Apply the processor
7. Download the simulation as a .mrsim file
8. Replacing PATHNAME with downloaded file pathname run the following code

import mrsimulator

sim, processors, _ = mrsimulator.load("PATHNAME")
# sim.config.decompose_spectrum = "spin_system"
sim.run()

for processor, method in zip(processors, sim.methods):
    processed_data = processor.apply_operations(method.simulation).real

Expected behavior:
Downloaded data is simulated and processed without error

Observed behavior:
The last line throws IndexError: list index out of range error

Note: If line 4 is uncommented then the code runs without error