Add optimal argument to Phono3py.save_mlp

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[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.52%. Comparing base (5d8f6ea) to head (2fec8a1).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
phono3py/api_phono3py.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #600   +/-   ##
=======================================
  Coverage   78.52%   78.52%           
=======================================
  Files         122      122           
  Lines       19400    19400           
=======================================
  Hits        15234    15234           
  Misses       4166     4166           

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