Update subsection for random uniform particle generator

[gassmoeller]

This is in response to https://community.geodynamics.org/t/cannot-specify-the-number-of-particles-with-random-uniform-particle-generator/3709/2. It was a mistake to let the 'random uniform' particle generator use the same subsection as the 'probability density function'. I think this was a leftover, because 'random uniform' used to be a child class of 'probability density function'. The change is straightforward, except the update script implementation is a bit complicated, because it needs to handle two cases:

1. .prm files older than the update of the particle parameters need to move 'Number of particles' into the new 'Random uniform' subsection.
2. .prm files younger than the update (or which have already been updated) need to move the whole subsection of 'Probability density function' to the new 'Random uniform' subsection.

[bangerth]

Yes, this is the right time to change this!

I think you might want to add a changelog entry. OK to merge if you do!

[bangerth]

Ah, it tests correctly. Let's just merge. Please add the changelog separately.