Add Triangle.ffill()

[genedan]

Description

Pandas DataFrame.ffill() can be mirrored to fill NaNs in a forward direction along the specified axis.

Leaving this as a proposed "Great First Contribution" as this would be easier than the proposed interpolation/extrapolation feature.

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.

tri = cl.Triangle(
    data={
        'origin': [1985, 1985, 1985, 1985, 1986, 1986, 1986, 1987, 1987, 1988],
        'development': [1985, 1986, 1987, 1988, 1986, 1987, 1988, 1987, 1988, 1988],
        'paid': [500, np.nan, 700, np.nan, np.nan, 1000, 1100, 1200, 1300, np.nan]
    },
    origin='origin',
    development='development',
    columns=['paid'],
    cumulative=True
)
tri

          12      24      36  48
1985   500.0     NaN   700.0 NaN
1986     NaN  1000.0  1100.0 NaN
1987  1200.0  1300.0     NaN NaN
1988     NaN     NaN     NaN NaN

tri.ffill(axis=3)

          12      24      36     48
1985   500.0   500.0   700.0  700.0
1986     NaN  1000.0  1100.0 1100.0
1987  1200.0  1300.0     NaN    NaN
1988     NaN     NaN     NaN    NaN

tri.ffill(axis=4)
          12      24      36  48
1985   500.0     NaN   700.0 NaN
1986   500.0  1000.0  1100.0 NaN
1987  1200.0  1300.0     NaN NaN
1988  1200.0     NaN     NaN NaN

### Do you have any additional supporting notes?

_No response_

### Would you be willing to contribute this ticket?

- [ ] Yes, absolutely!
- [ ] Yes, but I would like some help.
- [ ] No.