Enforce non-negative percents for material.add_nuclide to prevent unintended ao/wo flipping

[johvincau]

Description

In the Python API, when adding nuclides to a material, there appears to be an unintentional second way to flip between at% and wt% definitions as was discovered in #2669.

Behind the scenes and on the C++ side of things, OpenMC differentiates between at% and wt% inputs with sign, with negatively signed values indicating the latter. Thus, in the Python API, when the add_nuclide method is called with 'wo', the input percentage later has its sign flipped when running OpenMC.

However, what this means is that material.add_nuclide(nuc, -(percent), 'wo') is interpreted as an at% input, which can lead to the unintentional mixing of 'ao' and 'wo' for a material if by chance a stray negative sign were entered. This throws an error at runtime that fails to indicate that all add_nuclide percent values should be positive for composition type consistency.

Quick Fix

Simply requiring a non-negative value when calling add_nuclide makes the fix much more apparent by returning a negative ValueError when the given percent is negative.

While there's a chance some users manually utilize the negative percent to toggle between ao and wo, it's clear this is undocumented and likely unintentional behavior, and it's more transparent to just require 'ao' or 'wo' for controlling percent composition type.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[paulromano]

Thanks for addressing this! Can you do the same for the add_element method?

[johvincau]

Added! Let me know if there's anything else