Interpolate a/b Parameters for ctac_to_mu Conversion Based on keV/kvP

[samdporter]

Description:

Currently, when using a slopes file (e.g., ct_slopes.json), the conversion from CT attenuation coefficients (ctac) to μ values relies on a hard-coded keV/kvP value.

Feature Request:

Implement interpolation for the a/b parameters in the slopes file. Specifically, for any given keV/kvP value, the conversion function should interpolate between the nearest a and b values defined in the slopes file, and then use the interpolated parameters to perform the ctac to μ conversion.

Example:

If slopes.json contains entries for keVvalues of 50 and 100 with corresponding (a, b), then for an input of 75 keV, the conversion should linearly interpolate between these pairs to compute the effective parameters.