Fix Problem in merge

[LukasFehring]

During merging the "For Update" functionality stopped being used in mysql. I also added a test case

Description

Use For Update again

Motivation and Context

Experiments got pulled multiple times

Changes

The function _get_pull_experiment_query was not used after the merge. I put it back into the code. I also added a test case, that raises an error if an experiment is pulled multiple times.

Type Of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

How has This Been Tested?

• Database provider: mysql
• Python version: Any
• Operating System: Any

Does this Close/Impact Existing Issues?

Solves #174

What is Missing?

Checklist

• My change is based on the latest stage of the develop branch.
• My change required a change of the documentation, which has been done.
• I checked that the documentation can be build, visualizes everything as expected, and does not contain any warnings.
• I have added/adapted tests to cover my changes.
• The tests can be executed successfully.
• The notebooks can be executed successfully.
• The notebooks can be executed with mysql as provider.
• I have added a description of the changes to CHANGELOG.rst.

[tornede]

LGTM