Transient background estimation

[Yong2Sheng]

This PR adds the functionality of estimating the background for transients. It averages the background windows on the two sides of the burst and scale it to the burst window.

Now it only supports scaling by burst duration. We can also fit the burst background level in a light curve using the background windows first. Then, use the fitted result to estimate the number of background counts in the burst interval. This scaling method can be supported after merging #239

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.81%. Comparing base (5fe8fd8) to head (30cec2d).

Files with missing lines	Coverage Δ
cosipy/__init__.py	100.00% <100.00%> (ø)
...ground_estimation/TransientBackgroundEstimation.py	100.00% <100.00%> (ø)
cosipy/background_estimation/__init__.py	100.00% <100.00%> (ø)

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[Yong2Sheng]

The new changes are:

1. Move the scaling parameter from initializing TransientBackgroundEstimation to function make_background. So you do not need to define another class when you want to use a different scaling method.
2. Add unit test for TransientBackgroundEstimation. It has 100% coverage now.