Hard-coded prompt fragments

[jvm123]

It should be possible to overwrite hard-coded prompt fragments. This mostly concerns a few strings in PromptSampler._identify_improvement_areas().

One of them is:
https://github.com/codelion/openevolve/blob/5878857abe736d4f528d4fd834ca11e8d112fa3f/openevolve/prompt/sampler.py#L171

[jvm123]

This is related to issue #67, but not quite the same.

[codelion]

We can potentially parameterize the prompts move them to templates.py and change them as needed.