Add chain parameter to Material.get_activity for half-life data

[EdenRochmanSharabi]

Summary

• Add optional chain parameter to Material.get_activity() and Results.get_activity() that accepts a depletion chain (object or path to XML)
• When provided, half-life values from the chain are preferred over the default ENDF/B-VIII.0 values
• Nuclides not found in the chain fall back to the default openmc.data.decay_constant()

Test plan

• Verify get_activity() without chain parameter produces identical results to current behavior
• Verify get_activity(chain=chain) uses chain half-life values
• Verify fallback to default values for nuclides not in the chain
• Verify Results.get_activity(chain=...) passes the parameter through correctly

Closes #3529

[paulromano]

Thanks for the PR @EdenRochmanSharabi! I ended up doing a bit of refactoring here, namely extending half_life and decay_constant to take the chain_file arguments to make things more explicitly. I've also introduced the following convention:

• chain_file=False means "use ENDF data"
• chain_file=None means "use the default in openmc.config['chain_file']
• chain_file=x means use the chain from path or Chain object x

For half_life and decay_constant, the default is chain_file=False to preserve the existing behavior. For Material and Results, the get_activity methods default to chain_file=None to use whatever chain file is configured.

@shimwell I'd appreciate a second opinion on my design here. Does it seem reasonable to you?