Fix Legendre coefficients in AngleDistribution.from_endf

[paulromano]

Description

While working on another feature, I noticed that the Legendre coefficients read in AngleDistribution.from_endf are wrong. This happens because of the line [1.0] + al where al are the higher-order Legendre coefficients. It used to be a list so [1.0] + al made sense but when we switched to the endf module, al is now a numpy array so doing [1.0] + al adds 1 to every entry by array broadcasting. The updated code fixes that.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) 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)

[GuySten]

Looks good to me.