Reconsider nomenclature of `EITData` properties concerning pixel impedance

[DaniBodor]

Do these properties have the standard nomenclature in the field, or did you come up with them for this module? To me they are a bit confusing, but if this is what they are generally known as, then it's fine.

Originally posted by @DaniBodor in #142 (comment)

eitprocessing/eitprocessing/eit_data/__init__.py

Lines 267 to 291 in 022060c

	@property
	def pixel_impedance_global_offset(self) -> np.ndarray:
	"""Return the pixel impedance with the global baseline removed.
	In the resulting array the minimum impedance across all pixels is set to 0.
	"""
	return self.pixel_impedance - self.global_baseline
	@property
	def pixel_baseline(self) -> np.ndarray:
	"""Return the lowest value in each individual pixel over time."""
	return np.nanmin(self.pixel_impedance, axis=0)
	@property
	def pixel_impedance_individual_offset(self) -> np.ndarray:
	"""Return the pixel impedance with the baseline of each individual pixel removed.
	Each pixel in the resulting array has a minimum value of 0.
	"""
	return self.pixel_impedance - self.pixel_baseline
	@property
	def global_impedance(self) -> np.ndarray:
	"""Return the global impedance, i.e. the sum of all pixels at each frame."""
	return np.nansum(self.pixel_impedance, axis=(1, 2))

[psomhorst]

These were removed when moving global impedance to ContinuousData.