MaterialParameters object is mutated by calculation

[oashour]

Reusing the same MaterialParameters object for multiple calculations mutates it, causing an assertion error.

Traceback (most recent call last):
  File "/pscratch/sd/a/ash/He/hcp/pres_sweep/calc_darkmagic.py", line 44, in <module>
    material = PhononMaterial(material_name, params, yaml_file)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/global/u1/a/ash/code/DarkMAGIC/darkmagic/material.py", line 271, in __init__
    properties.validate_for_phonons(n_atoms)
  File "/global/u1/a/ash/code/DarkMAGIC/darkmagic/material.py", line 90, in validate_for_phonons
    assert any(np.any(v) for v in d.values())