Kernels should be pure

### Are there any linked Issues or Pull Requests?

_No response_

### Brief description

Once kernels are independent of _config_mod settings, we should be able to make them pure to prevent inadvertent changes that reverse their status. This can be done as a change to existing pure kernels and documented as part of the standard for writing kernels.

### Further details of the issue.

_No response_

Start by locating existing pure kernels and their references to _config_mod settings, then review the standard for writing kernels. Done means the relevant kernels are independent of those settings and the purity requirement is documented in the kernel-writing standard.

Written by the indexing model from the issue text.