Add a function that fills NaN in a triangle with 0

[henrydingliu]

Description

fillna(0) doesn't do this. Not sure if intentional.

Is your feature request aligned with the scope of the package?

• Yes, absolutely!
• No, but it's still worth discussing.
• N/A (this request is not a codebase enhancement).

Describe the solution you'd like, or your current workaround.

Not sure if we should change fillna(0) or make a new function.

fillna(0) is used a few times elsewhere in the package.

Do you have any additional supporting notes?

run the following to see

raa = cl.load_sample('raa')
print(raa-raa)
print((raa-raa).fillna(0))
print((raa-raa).fillna(1))