Change 'np.NaN' to 'np.nan' re: Numpy 2.0

[douglatornell]

'numpy.nan' is the correct spelling in Numpy 2.0. All others are deprecated.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 77.64%. Comparing base (4d01c91) to head (6296683).
Report is 27 commits behind head on main.

Files with missing lines	Patch %	Lines
nowcast/figures/research_ferries.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #272   +/-   ##
=======================================
  Coverage   77.64%   77.64%           
=======================================
  Files         133      133           
  Lines       18695    18695           
  Branches     1910     1910           
=======================================
  Hits        14515    14515           
  Misses       4113     4113           
  Partials       67       67           

Flag	Coverage Δ
unittests	77.64% <0.00%> (ø)

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