Replace n_traj with ntraj for QuTiP compatibility

[albertomercurio]

Description

QuTiP uses ntraj as an argument of Monte Carlo and stochastic simulations. Here I change the current argument from n_traj to ntraj, to align to QuTiP syntax.

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 93.51%. Comparing base (bdac5f4) to head (db5d485).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/time_evolution/mcsolve.jl	75.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #243   +/-   ##
=======================================
  Coverage   93.51%   93.51%           
=======================================
  Files          31       31           
  Lines        2268     2268           
=======================================
  Hits         2121     2121           
  Misses        147      147           

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[ytdHuang]

LGTM!