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etable: exponential tabular model

An exponential table model. The filename to be used should be given immediately after etable in the model command. For example: 

XSPEC> model etable{mymod.mod}

uses mymod.mod as the input for the model. XSPEC will multiply the contents of the model by -1 then take the exponential i.e. this model is for calculating absorption functions. For specifications of the table model file, see the OGIP memo 92-009 on the FITS file format for table model files (available from https://heasarc.gsfc.nasa.gov/docs/heasarc/ofwg/docs/summary/ogip_92_009_summary.html).

The model value is assumed unity outside the tabulated energies unless the keywords LOELIMIT and HIELIMIT are given in the primary extension of the FITS file. In the latter case the value given in LOELIMIT will be used for energies below those tabulated and HIELIMIT for energies above those tabulated.

A table model file may also provide a neural network (NN) emulation of the tabulated model, indicated by the NNPTFILE keyword in the primary extension pointing to a PyTorch (.pt) weights file. When such a file is loaded XSPEC automatically evaluates the model with the neural network instead of interpolating on the parameter grid; this requires XSPEC to have been built with libtorch support. See atable for details.