Update around variable names

[hyrodium]

• Update the name of field from l to skindepth
• Update the name of field from Γ to inertia
• Update the name of field from ε to emissivity
• Update the name of field from P to period
• Rename nfaces to n_face
• Rename nsteps to n_step
• Rename nsteps_to_save to n_step_to_save
• Rename nsteps_in_cycles to n_step_in_cycle
• Rename ncycles to n_cycle

[codecov]

Codecov Report

Attention: Patch coverage is 97.43590% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 94.28%. Comparing base (d0ebe8e) to head (bc14e13).
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/TPM.jl	94.11%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #134   +/-   ##
=======================================
  Coverage   94.28%   94.28%           
=======================================
  Files          10       10           
  Lines         683      683           
=======================================
  Hits          644      644           
  Misses         39       39           

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