Add convoluted Gaussian and Exponential Component

[jlaehne]

Description of the change

Add a component that contains the convolution of an exponential decay with a Gaussian instrument response function.

Progress of the PR

• Change implemented (can be split into several points),
• docstring updated (if appropriate),
• update user guide (if appropriate),
• added tests,
• add a changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• Check formatting of the changelog entry (and eventual user guide changes) in the docs/readthedocs.org:lumispy build of this PR (link in github checks),
• ready for review.

Minimal example of the bug fix or the new feature

import lumispy as lum
m = s.create_model()
g1 = lum.components.ConvGaussianExponential()
m.append(g1)
m.fit()

[jlaehne]

@aidanc151 example of adding a component to lumispy :-)

[ericpre]

It should be good to add this component to the document - I was looking for it to check for the rendering of the analytical expression!

[ericpre]

Add a test that check that there is no normalisation issue: for example by making a dataset where the Gaussian convolution is negligible and checking that the fitting parameter are very close for the ConvGaussExp and Exponential components.

[ericpre]

I didn't check the math but from a quick look (based on https://math.stackexchange.com/questions/685041/convolute-exponential-with-a-gaussian/872561#872561), it seems that it should be a "-"?

Suggested change

	(1 + erf(((x-t0) - sigma**2/tau)/(sqrt(2)*sigma)))",
	(1 - erf(((x-t0) - sigma**2/tau)/(sqrt(2)*sigma)))",

Could it be replaced by the erfc function?

[ericpre]

Add a reference where the math comes from?
For example, https://math.stackexchange.com/questions/685041/convolute-exponential-with-a-gaussian/872561#872561 has good pointers?

[jlaehne]

So far, it is based on https://zenodo.org/records/6384277 - but we are checking the math again with mathematica. As perspective we will at least add a component with double exponential as well.

[ericpre]

Out of curiosity, are you using mathematica because sympy can't do it or just haven't tried with sympy?

[jlaehne]

A student already did the calculations in mathematica and I did not have the time yet to copy that in sympy :-)