fix calculate_implied_rate to scale according to position duration

[MazyGio]

PR originally from delvtech/hyperdrive#1006

Resolved Issues

Fixes delvtech/hyperdrive/#1003

Description

Fixes calculate_implied_rate to adjust the variable rate according to the term duration and compounding frequency.

We should probably add a test with a position duration different than 1 year, but I couldn't easily figure out how to.

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