Add get_decay_heat() function to deplete.Results

[kevinm387]

Description

Hello, this PR adds a function to get the decay heat of a material at different steps of a depletion run. It follows a similar pattern as the other functions of deplete.Results, especially the get_activity() function.

I also noticed a mistake in the deplete.Results.get_activity() docstring, so this PR corrects that as well.

Please let me know what you think, thanks!

Fixes # (issue)

Checklist

• I have performed a self-review of my own code
• I have run clang-format 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 @kevinm387!