Allow finding best emodel without storing it.

[eleftherioszisis]

Move the functionality for finding the best emodel to its own function. That allows separation of concerns, especially in cases where the access_point is not used to register the emodel.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
bluepyemodel/optimisation/optimisation.py	50.00%	2 Missing ⚠️

Files with missing lines	Coverage Δ
bluepyemodel/optimisation/optimisation.py	25.80% <50.00%> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[darshanmandge]

Thanks!
We can keep this open for a while to see if it is needed for the export task.