Passing unit_symbol to prevent nonsense prefixes

[Nathan-Majernik]

nice_scale_prefix(), nice_array(), and plottable_array() now all optionally take a unit_symbol argument. This allows the detection of prefixes that don't make sense and prevents them from being used. Other code which calls these functions in the repo updated to reflect the change

Fixes #116

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Basic test:

python -c "
from pmd_beamphysics.units import nice_scale_prefix

# Test the updated cases
print('Test: A (Ampere) unit should get prefix')
f, p = nice_scale_prefix(1e-6, unit_symbol='A')
print(f'  Result: factor={f}, prefix=\"{p}\"')

print('\nTest: √m unit should NOT get prefix')
f, p = nice_scale_prefix(1e-6, unit_symbol='√m')
print(f'  Result: factor={f}, prefix=\"{p}\"')

print('\nTest: m (meter) unit should get prefix')
f, p = nice_scale_prefix(1e-6, unit_symbol='m')
print(f'  Result: factor={f}, prefix=\"{p}\"')
"