Fix a bug in converting units for List[System]

[GardevoirX]

In _convert_systems_units, we calculate the unit conversion factor, conversion, for length, and then we enter a for-loop, to apply the conversion to systems:

metatomic/python/metatomic_torch/metatomic/torch/model.py

Lines 933 to 949 in 475fab2

	if model_length_unit == "" or system_length_unit == "":
	# no conversion for positions/cell/NL
	conversion = 1.0
	else:
	conversion = unit_conversion_factor(
	quantity="length",
	from_unit=system_length_unit,
	to_unit=model_length_unit,
	)
	new_systems: List[System] = []
	for system in systems:
	new_system = System(
	types=system.types,
	positions=conversion * system.positions,
	cell=conversion * system.cell,
	pbc=system.pbc,

However, when there are additional inputs attached to a system, to convert them to our requested unit, we calculate the conversion factor, and also store it in conversion:

metatomic/python/metatomic_torch/metatomic/torch/model.py

Lines 976 to 983 in 475fab2

	if requested.quantity != "" and unit is not None:
	conversion = unit_conversion_factor(
	quantity=requested.quantity,
	from_unit=unit,
	to_unit=requested.unit,
	)
	else:
	conversion = 1.0

This covers the original conversion factor, and in the next round of for-loop, we are using a wrong length conversion factor.

This is fixed by renaming the conversion factor used for length to length_conversion, but I am not sure if it's the best solution, and if we need to add some tests to prevent this from happening again in future

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