Fixes ``tstart`` not updating in ``stingray.Lightcurve.shift``

[ankitkhushwaha]

fixes issues #911, #916

Implemented all the required changes.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.77%. Comparing base (cc4c259) to head (514e5e3).

Files with missing lines	Patch %	Lines
stingray/lightcurve.py	83.33%	2 Missing :warning:

:exclamation: There is a different number of reports uploaded between BASE (cc4c259) and HEAD (514e5e3). Click for more details.

HEAD has 2 uploads less than BASE

Flag	BASE (cc4c259)	HEAD (514e5e3)
	5	3

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #912       +/-   ##
===========================================
- Coverage   91.53%   79.77%   -11.76%     
===========================================
  Files          48       48               
  Lines        9770     9782       +12     
===========================================
- Hits         8943     7804     -1139     
- Misses        827     1978     +1151     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Sauravroy34]

Hi, after looking up i found that there are few methods which supports gaussian error distributions

1. Multitaper for spectral density estimation

2. Stingray.simulator supports gaussian error distrubtions

3. Lightcurve.read assumes gaussian error by default

4. Powerspectrum too supports gaussian error distributions

5. In Crossspectrum api flux generator supports