ExponentialParticleDistribution: Use rate-based formula

[leroyvn]

Description

This PR changes the exponential particle distribution function to a rate-based formula. The original formula

$f : x \mapsto \dfrac{1}{\beta} \exp \left( - x / \beta \right)$

does not feel natural to all users (including myself), who may prefer the rate-based formula

$f : x \mapsto \dfrac{\lambda e^{-\lambda x}}{1 - e^{-\lambda}}$.

This PR still keeps the option of initialising this function with a scale parameter.

Checklist

• The code follows the relevant coding guidelines
• The code generates no new warnings
• The code is appropriately documented
• The code is tested to prove its function
• The feature branch is rebased on the current state of the main branch
• I updated the change log if relevant
• I give permission that the Eradiate project may redistribute my contributions under the terms of its license

[nollety]

Looks perfectly fine to me, just a few comments 😃
Since the formula was not incorrect per say, I'd change the PR title to something like:
ExponentialParticleDistribution: change formulation to rate-based