Bug causes warning on reading input file

[mikekryjak]

When reading an input file with BoutOptionsFile, the method recalculate_xyz is called during class init and fails:

inputfile = BoutOptionsFile(inputfile_path, gridfilename = "/home/mike/work/hermes3v/builds/newbuild/tests/integrated/dmplex/example_cdn.grd.nc")

/home/mike/work/hermes3v/views/gcc/lib/python3.11/site-packages/boutdata/data.py:765: AlwaysWarning: While building x, y, z coordinate arrays, an exception occured: 'float' object cannot be interpreted as an integer
Evaluating non-scalar options not available
  alwayswarn(

Digging deeper shows that it's caused by np.linspace getting a float instead of an integer:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[35], [line 1](vscode-notebook-cell:?execution_count=35&line=1)
----> [1](vscode-notebook-cell:?execution_count=35&line=1) inputfile.recalculate_xyz()

File ~/work/hermes3v/views/gcc/lib/python3.11/site-packages/boutdata/data.py:846, in BoutOptionsFile.recalculate_xyz(self, nx, ny, nz)
    842 myg = self._keys.get("MYG", 2)
    844 # make self.x, self.y, self.z three dimensional now so
    845 # that expressions broadcast together properly.
--> [846](https://vscode-remote+ssh-002dremote-002b7b22686f73744e616d65223a225030373238227d.vscode-resource.vscode-cdn.net/home/mike/work/notebooks/vantage/~/work/hermes3v/views/gcc/lib/python3.11/site-packages/boutdata/data.py:846) self.x = numpy.linspace(
    847     (0.5 - mxg) / (self.nx - 2 * mxg),
    848     1.0 - (0.5 - mxg) / (self.nx - 2 * mxg),
    849     self.nx,
    850 )[:, numpy.newaxis, numpy.newaxis]
    851 self.y = (
    852     2.0
    853     * numpy.pi
   (...)
    858     )[numpy.newaxis, :, numpy.newaxis]
    859 )
    860 self.z = (
    861     2.0
    862     * numpy.pi
   (...)
    865     ]
    866 )

File ~/work/hermes3v/views/gcc/lib/python3.11/site-packages/numpy/core/function_base.py:122, in linspace(start, stop, num, endpoint, retstep, dtype, axis)
     24 @array_function_dispatch(_linspace_dispatcher)
     25 def linspace(start, stop, num=50, endpoint=True, retstep=False, dtype=None,
     26              axis=0):
     27     """
     28     Return evenly spaced numbers over a specified interval.
     29 
   (...)
    120 
    121     """
--> [122](https://vscode-remote+ssh-002dremote-002b7b22686f73744e616d65223a225030373238227d.vscode-resource.vscode-cdn.net/home/mike/work/notebooks/vantage/~/work/hermes3v/views/gcc/lib/python3.11/site-packages/numpy/core/function_base.py:122)     num = operator.index(num)
    123     if num < 0:
    124         raise ValueError("Number of samples, %s, must be non-negative." % num)

TypeError: 'float' object cannot be interpreted as an integer

As you can see, nx is specifically loaded as a float:

boutdata/src/boutdata/data.py

Lines 819 to 821 in 7164a89

	with DataFile(gridfilename) as gridfile:
	self.nx = float(gridfile["nx"])
	self.ny = float(gridfile["ny"])

And here is the bit where it should've been an integer:

boutdata/src/boutdata/data.py

Lines 840 to 846 in 7164a89

	# make self.x, self.y, self.z three dimensional now so
	# that expressions broadcast together properly.
	self.x = numpy.linspace(
	(0.5 - mxg) / (self.nx - 2 * mxg),
	1.0 - (0.5 - mxg) / (self.nx - 2 * mxg),
	self.nx,
	)[:, numpy.newaxis, numpy.newaxis]

This seems like a very obvious fix, but I'm a bit hesitant, because I can't see how other users could have avoided getting the same error.
I appreciate this is probably very old code by now, but maybe someone has some ideas about this.
If not, I can just issue a PR to do the simple fix.