Fix wrong computations of Fourier series

[sylee957]

References to other Issues or PRs

Fixes #19375
Fixes #19044

Brief description of what is fixed or changed

If the center of the integration limits are not origin, detecting whether the function is even or not is more complicated, and it may not even be worth testing it out because substituting x to some shifted values of x can have the issues of equality.

Other comments

I've also made the docs clear about the definitions because it was unclear how limits is supposed to do, and made some plotting examples for it.

Release Notes

• series
  • Fixed wrong computations of fourier_series for even or odd functions with limits that are specified non-central.

[sympy-bot]

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• series
  • Fixed wrong computations of fourier_series for even or odd functions with limits that are specified non-central. (#19382 by @sylee957)

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#### References to other Issues or PRs
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Fixes #19375
Fixes #19044 


#### Brief description of what is fixed or changed

If the center of the integration limits are not origin, detecting whether the function is even or not is more complicated, and it may not even be worth testing it out because substituting `x` to some shifted values of `x` can have the issues of equality.

#### Other comments

I've also made the docs clear about the definitions because it was unclear how limits is supposed to do, and made some plotting examples for it.

![image](https://user-images.githubusercontent.com/34944973/82438764-88f93880-9ad4-11ea-8266-af5255d6daaf.png)

#### Release Notes

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<!-- BEGIN RELEASE NOTES -->
- series
  - Fixed wrong computations of `fourier_series` for even or odd functions with `limits` that are specified non-central.
<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[codecov]

Codecov Report

Merging #19382 into master will increase coverage by 0.147%.
The diff coverage is 100.000%.

@@              Coverage Diff              @@
##            master    #19382       +/-   ##
=============================================
+ Coverage   75.618%   75.766%   +0.147%     
=============================================
  Files          651       652        +1     
  Lines       169418    170795     +1377     
  Branches     39973     40322      +349     
=============================================
+ Hits        128112    129406     +1294     
- Misses       35703     35774       +71     
- Partials      5603      5615       +12     

[oscarbenjamin]

Looks good

[mfripp]

Thanks! This will help a lot if I start teaching Fourier series with sympy in SageMath, CoCalc or Jupyter.