Add n_e_separatrix field to pulse_schedule.density_control for time-dependent control of separatrix density

[fmunguia-wulf]

Implements @orso82's suggestion from ProjectTorreyPines/FUSE.jl#943:

• Add dd.pulse_schedule.density_control.n_e_separatrix field

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 43.97%. Comparing base (dd8bcbc) to head (72e5690).

Files with missing lines	Patch %	Lines
src/dd.jl	87.50%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #75   +/-   ##
=======================================
  Coverage   43.96%   43.97%           
=======================================
  Files          15       15           
  Lines       31419    31426    +7     
=======================================
+ Hits        13813    13819    +6     
- Misses      17606    17607    +1     

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