add zero-noise PE example for LISA

[WuShichao]

Standard information about the request

This is a: doc update

This change affects: inference, doc

This change changes: documentation

Motivation

In the current PyCBC Doc, it seems there are no zeroNoise examples for PE. Also, someone asked how to perform LISA PE with zero noise injection while still passing extra arguments to PSD functions in the likelihood calculation.

Contents

Add code to show how to use psd-extra-args with psd-model, when fake-strain = zeroNoise.

Testing performed

zero noise noise

Additional notes

• [x] The author of this pull request confirms they will adhere to the code of conduct

[WuShichao]

@ahnitz Can we merge it?

[ahnitz]

@WuShichao Shouldn't it be a separate example rather than overwriting the existing one? I know you left stuff in the comments, but I think a separate file would be better. Otherwise, if this works after that then you are good to go.