Tensorflow-NUFFT density is not density_compensation

[chaithyagr]

mri-nufft/src/mrinufft/operators/interfaces/tfnufft.py

Lines 48 to 51 in 8e15f05

	if density is True:
	self.density = tfmri.estimate_density(
	samples, shape, method="pipe", max_iter=15
	)

Here, estimate_density just estimates the density, not the density_compensators. What we need is a reciprocal of this values.

All we need to do is to change this line to:
tf.math.reciprocal_no_nan(tfmri.estimate_density(
samples, shape, method="pipe", max_iter=15
))

Also, some tests on this would be helpful.