Add inverse field distortion for nT

[ramirezdiego]

Issue to not lose track of this open point. The approach now for simulations is of course to not apply any FDC, since there's no distortion to correct for. But this can be reversed.

Inspiration can be taken from these lines, used for 1T (thanks @petergaemers for sharing):

WFSim/wfsim/core.py

Lines 708 to 731 in b204045

	def inverse_field_distortion(x, y, z, resource):
	"""For 1T the pattern map is a data driven one so we need to reverse engineer field distortion
	into the simulated positions
	:param x: 1d array of float
	:param y: 1d array of float
	:param z: 1d array of float
	:param resource: instance of resource class
	returns z: 1d array, postions 2d array
	"""
	positions = np.array([x, y, z]).T
	for i_iter in range(6): # 6 iterations seems to work
	dr = resource.fdc_3d(positions)
	if i_iter > 0:
	dr = 0.5 * dr + 0.5 * dr_pre # Average between iter
	dr_pre = dr
	r_obs = np.sqrt(x**2 + y**2) - dr
	x_obs = x * r_obs / (r_obs + dr)
	y_obs = y * r_obs / (r_obs + dr)
	z_obs = - np.sqrt(z**2 + dr**2)
	positions = np.array([x_obs, y_obs, z_obs]).T
	positions = np.array([x_obs, y_obs]).T
	return z_obs, positions