Added method to smothen lightcurve

[Sauravroy34]

This PR adds method to smoothen light curves using Poission regressor with spline transformation
closes #905

[Sauravroy34]

Updated after using poission regressor

[matteobachetti]

Hi @Sauravroy34, thanks for your contribution.

A couple of comments:

1. Please take a look at the contribution guidelines: https://docs.stingray.science/en/stable/contributing.html#coding-guidelines. In particular:
   a) We need tests for all functions, and for all lines of code. Tests should be as precise as possible, e.g. testing an expected output, not just that the function executes. Also, Exceptions and warnings should be tested.
   b) Check the style guide, black is very helpful for most of the requirements.

2. The methods you are listing now assume Gaussian, or at least symmetric, statistical distributions for the counts arrays. These will probably fail for low-countrate data. Among smoothing methods, since light curves are usually Poisson-distributed, it would be good to add a method specific to Poisson-distributed data. Do you have any good one in mind?

[Sauravroy34]

i wil try to modiify this code using sckit learn poissonregressor