Allow Triangle.drop() to drop origin levels

[genedan]

Description

Triangle.drop() should allow for the dropping of origin levels when axis=2.

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='1987', axis='2')

         12     24     36
1985  300.0  400.0  500.0
1986  500.0  600.0    NaN

Do you have any additional supporting notes?

Dropping of labels that are not the first or last origin period should not be allowed.

Would you be willing to contribute this ticket?

• Yes, absolutely!
• Yes, but I would like some help.
• No.