Follow default IMP: Cell Importance logic

[tjlaboss]

Is your feature request related to a problem? Please describe.

The MCNP 6.3.1 Manual § 5.12.1 describes the input syntax for imp: (cell importance) cards. Default cell importances follow some logic described in the proceeding paragraphs.

The default particle importance for a cell (if any) depends on a number of factors, including:

• Other particle importances on the cell
• Presence of a WWN card
• Cell-based vs. mesh-based weight windows
• Whether importance is not set, or explicitly skipped with the Jump feature.

Describe the solution you'd like
Implement the logic described in the MCNP6.3.1 manual and set cell importances accordingly.

Describe alternatives you've considered
Unset cell importances resulted in fatal errors in MontePy, and this is better than a partial implementation that could set incorrect values.

Additional context
See Issue #381 and PR #656