Allow Triangle.drop() to drop development periods

[genedan]

Description

Triangle.drop() should be able to drop a development period when axis=3.

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, 1986, 1986, 1987],
        'development': [1985, 1986, 1987, 1986, 1987, 1987],
        'paid': [300, 400, 500, 500, 600, 500],
    },
    origin='origin',
    development='development',
    columns=['paid'],
    cumulative=True
)
print(tri)

         12     24     36
1985  300.0  400.0  500.0
1986  500.0  600.0    NaN
1987  500.0    NaN    NaN

print(tri.drop(labels='12`, axis=3)

          24     36
1985   400.0  500.0
1986   600.0    NaN
1987     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.