[GSoC] Functions: Fixes limit evaluations involving error functions

[sachin-4099]

Fixes: #8433
Fixes: #10976
Fixes: #11496
Fixes: #13733
Fixes: #13750

Brief description of what is fixed or changed

Rectifies _eval_rewrite_as_tractable method of class erf.

Adds eval method to class _erfs so that _erfs(0) evaluates to 1.

Corrects the _eval_as_leading_term method of class erf.

Other Comments

Regression Tests have been added.

Release Notes

• functions
  • Rectifies _eval_rewrite_as_tractable method of class erf

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• functions
  • Rectifies _eval_rewrite_as_tractable method of class erf (#19697 by @sachin-4099)

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Fixes: #8433
Fixes: #10976
Fixes: #11496
Fixes: #13733
Fixes: #13750

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

Rectifies `_eval_rewrite_as_tractable` method of `class erf`.

Adds `eval` method to `class _erfs` so that `_erfs(0)` evaluates to `1`.

Corrects the `_eval_as_leading_term` method of `class erf`.


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


#### Release Notes


<!-- BEGIN RELEASE NOTES -->
* functions
  * Rectifies `_eval_rewrite_as_tractable` method of `class erf`
<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[codecov]

Codecov Report

Merging #19697 into master will increase coverage by 16.120%.
The diff coverage is 100.000%.

@@              Coverage Diff               @@
##            master    #19697        +/-   ##
==============================================
+ Coverage   59.598%   75.719%   +16.120%     
==============================================
  Files          658       658                
  Lines       171259    171340        +81     
  Branches     40423     40450        +27     
==============================================
+ Hits        102068    129737     +27669     
+ Misses       63355     35940     -27415     
+ Partials      5836      5663       -173     

[jksuom]

Thanks, this looks good. Merging.