[GSoC] Functions: Fixes limit evaluations related to LambertW function

[sachin-4099]

Fixes: #18969

Brief description of what is fixed or changed

Adds _singularities to LambertW function.

Made minor changes to Pow._eval_nseries to have a more general series expansion.

Other Comments

Regression Tests have been added.

Release Notes

• functions
  • Adds _singularities to LambertW function

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• functions
  • Adds _singularities to LambertW function (#19716 by @sachin-4099)

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Fixes: #18969

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

Adds `_singularities` to `LambertW` function.

Made minor changes to `Pow._eval_nseries` to have a more general series expansion.


#### Other Comments
Regression Tests have been added.


#### Release Notes


<!-- BEGIN RELEASE NOTES -->
* functions
  * Adds `_singularities` to `LambertW` function
<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[sachin-4099]

Again Github has not updated to reflect the fact that the Travis tests are complete (#19589).

I'm hoping that this comment will cause it to update.

[codecov]

Codecov Report

Merging #19716 into master will increase coverage by 0.010%.
The diff coverage is 91.735%.

@@              Coverage Diff              @@
##            master    #19716       +/-   ##
=============================================
+ Coverage   75.693%   75.704%   +0.010%     
=============================================
  Files          658       659        +1     
  Lines       171307    171328       +21     
  Branches     40441     40414       -27     
=============================================
+ Hits        129669    129703       +34     
- Misses       35973     35981        +8     
+ Partials      5665      5644       -21     

[jksuom]

Thanks, this looks good.