added aliases for arc___ functions

[coquelin77]

Description

aliases added, tests rearranged into alphabetic order in test_trigonmetrics

Issue/s resolved: #628

Changes proposed:

• added aliases for arctan -> atan, arctan2 -> atan2, arccos -> acos, arcsin -> asin
• ordered tests in test_trig...

Type of change

• New feature (non-breaking change which adds functionality)

Due Diligence

• All split configurations tested
• Multiple dtypes tested in relevant functions
• Documentation updated (if needed)
• Updated changelog.md under the title "Pending Additions"

Does this change modify the behaviour of other functions? If so, which?

no

[codecov]

Codecov Report

Merging #629 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #629   +/-   ##
=======================================
  Coverage   97.49%   97.49%           
=======================================
  Files          77       77           
  Lines       15869    15877    +8     
=======================================
+ Hits        15472    15480    +8     
  Misses        397      397           

Impacted Files	Coverage Δ
heat/core/tests/test_trigonometrics.py	100.00% <100.00%> (ø)
heat/core/trigonometrics.py	96.15% <100.00%> (+0.69%)	⬆️

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