[GSoC] Functions: Fixes _eval_nseries() of log

[sachin-4099]

Fixes: #18842

Brief description of what is fixed or changed

Rewrites _eval_nseries method of log present in exponential.py.

Other Comments

Regression Test has been added.

Release Notes

• functions
  • Fixes _eval_nseries method of log

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• functions
  • Fixes _eval_nseries method of log (#20002 by @sachin-4099)

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Fixes: #18842

#### Brief description of what is fixed or changed
Rewrites `_eval_nseries` method of `log` present in `exponential.py`.

#### Other Comments
Regression Test has been added.

#### Release Notes


<!-- BEGIN RELEASE NOTES -->
* functions
  * Fixes `_eval_nseries` method of `log`
<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[codecov]

Codecov Report

Merging #20002 into master will increase coverage by 4.461%.
The diff coverage is 100.000%.

@@              Coverage Diff              @@
##            master    #20002       +/-   ##
=============================================
+ Coverage   71.411%   75.872%   +4.461%     
=============================================
  Files          669       669               
  Lines       173512    173546       +34     
  Branches     40956     40966       +10     
=============================================
+ Hits        123907    131674     +7767     
+ Misses       43742     36137     -7605     
+ Partials      5863      5735      -128     

[jksuom]

Thanks, I think this is ready to be merged.