Support intervalds and/or intervaldsus

**Is your feature request related to a problem? Please describe.**
It would be convenient to store time durations directly in the DB rather than storing it as a number of seconds.

**Describe the solution you'd like**
Use a Python timedelta to map to the Comdb2 intervalds or intervaldsus values. This seems like a natural mapping.

**Describe alternatives you've considered**
The alternative is to use a double or an int (depending on the needed precision) to store the duration as a number of seconds.

該 issue 提到了 Python timedelta 和 Comdb2 的 intervalds/intervaldsus 值，但沒有指定檔案或測試。首先定位現有的 Python-to-Comdb2 type mappings 及其測試，然後確定應如何將 timedelta 的精度映射到每種 interval 類型。完成標準是可以透過 API 儲存和取得 duration，而不使用數值秒數。

由索引模型根據 Issue 內容生成。