[GSoC] Core: Fixes _eval_nseries() of Power

[sachin-4099]

Fixes: #9549
Fixes: #12578
Fixes: #12769
Fixes: #15146
Fixes: #17751
Fixes: #18008

Brief description of what is fixed or changed

The series expansion of b**e is computed as follows:

• We express b as f*(1 + g) where f is the leading term of b.
  g has order O(x**d) where d is strictly positive.
• Then b**e = (f**e)*((1 + g)**e). (1 + g)**e is computed using binomial series.

It was very important to rewrite and clean-up Pow._eval_nseries() completely, so that many issues get resolved and it becomes easy to debug any further bugs related to series expansions or limit evaluations.

Other Comments

Regression Tests have been added.

Release Notes

• core
  • Fixes _eval_nseries() function of power.py

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• core
  • Fixes _eval_nseries() function of power.py (#19508 by @sachin-4099)

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Fixes: #9549
Fixes: #12578
Fixes: #12769
Fixes: #15146
Fixes: #17751 
Fixes: #18008

#### Brief description of what is fixed or changed
The series expansion of `b**e` is computed as follows:
* We express `b` as `f*(1 + g)` where `f` is the leading term of `b`. 
    `g` has order `O(x**d)` where `d` is strictly positive.
* Then `b**e` = `(f**e)*((1 + g)**e)`. `(1 + g)**e` is computed using binomial series.

It was very important to rewrite and clean-up `Pow._eval_nseries()` completely, so that many issues get resolved and it becomes easy to debug any further bugs related to series expansions or limit evaluations.

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

#### Release Notes


<!-- BEGIN RELEASE NOTES -->
* core
  * Fixes `_eval_nseries()` function of `power.py`
<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[codecov]

Codecov Report

Merging #19508 into master will increase coverage by 11.494%.
The diff coverage is 97.368%.

@@              Coverage Diff               @@
##            master    #19508        +/-   ##
==============================================
+ Coverage   64.186%   75.681%   +11.494%     
==============================================
  Files          652       653         +1     
  Lines       169748    169836        +88     
  Branches     40071     40047        -24     
==============================================
+ Hits        108956    128535     +19579     
+ Misses       54578     35689     -18889     
+ Partials      6214      5612       -602     

[jksuom]

Thanks, this is ready to be merged/