Preventing users from updating the box size

[Jean-Noël Grad]

Dear users,

When need your feedback on a significant change to ESPResSo 4.2.2.

When manually changing the system.box_l property, particles positions 
are incorrectly updated. We do not think we can address the bug in a way 
that would satisfy all users, because different features are affected in 
different ways. For example, increasing the box size while leaving the 
particle at their current position can change the number of clusters and 
introduce jumps in the energy curve (when non-bonded interactions are 
active). Folding and then unfolding coordinates with the new box size 
can introduce artifacts due to Lees-Edwards boundary conditions. 
Trajectories in unfolded coordinates and position auto-correlation 
functions can be affected too.

We think the easiest way to address this issue is to prevent users from 
changing the box_l value when particles are in the system. The box size 
can still be changed via `system.change_volume_and_rescale_particles()`, 
although with a significant performance penalty due to 3 consecutive 
retunings (one for each direction).

If you have written simulation scripts that change box_l while particles 
are still present in the system, and cannot adapt the code to use the 
rescale algorithm, please let us know on the mailing list or in issue 
#4834 (https://github.com/espressomd/espresso/issues/4834) until **March 
18, 2024**. If we do not hear from you until then, we will make the 
box_l non-writeable when particles are present in ESPResSo 4.2.2 and the 
development branch.

Best,
JN