Enhancement reshape rename axis to new_split and add description

[mtar]

Description

Renaming of the axis parameter and additional hint that it only works on split DNDarrays.

Issue/s resolved: #665

Changes proposed:

• rename axis to new_split
• add further description

Type of change

• Documentation update

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 #667 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #667   +/-   ##
=======================================
  Coverage   97.37%   97.37%           
=======================================
  Files          85       85           
  Lines       16646    16646           
=======================================
  Hits        16209    16209           
  Misses        437      437           

Impacted Files	Coverage Δ
heat/core/manipulations.py	99.12% <100.00%> (ø)
heat/core/tests/test_manipulations.py	99.91% <100.00%> (ø)

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[lenablind]

Thank you for adapting the function so quickly!