Add constant value setting for data elements in configuration.

We need a system that does something like:

```toml
[data.variable]
var_name = "precipitation"
constant = 4
```

I have a feeling that there is a sketch in a discussion somewhere!

The issue names no files, tests, or entry points and refers to an unspecified discussion; start by finding that discussion and tracing the existing configuration handling. Before implementation, establish the intended behavior for the TOML constant setting and identify the relevant tests. Done should mean the scope and expected behavior are agreed and covered by tests.

Written by the indexing model from the issue text.