Fix tutorial imports for installed package usage

[Copilot]

Tutorial scripts used development imports (from src.rcol...) that fail when the package is installed via pip with ModuleNotFoundError: No module named 'src'.

Changes

Import fixes

• Changed tutorial_existing_instruments.py imports from src.rcol to rcol namespace
• Verified tutorial_custom_instruments.py already uses correct imports

Documentation improvements

• Added docstring headers with prerequisites and usage instructions to both tutorials
• Added "Running Tutorial Scripts" section to README with pip installation steps
• Structured tutorial code with section markers for clarity

Error handling

• Replaced bare Exception catches with specific RedcapError
• Added empty list check before pd.concat() in RTG instrument collection
• Standardized RC_API_KEY environment variable usage

Example

Before:

from src.rcol.instruments import fal, ehi, bdi_ii
import src.rcol.rtg as rtg

After:

from rcol.instruments import fal, ehi, bdi_ii
import rcol.rtg as rtg

Users can now run tutorials directly after pip install rcol.

Original prompt

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This section details on the original issue you should resolve

<issue_title>error when using tutorial for existing instruments</issue_title>
<issue_description>https://github.com/JuliusWelzel/rcol/blob/3937810f31b0fc6fa571115033e5e6fde876a11d/tutorial_existing_instruments.py#L5C6-L5C9
When using the tutorial for regular application (not development) this code throws an error.

ModuleNotFoundError: No module named 'src'
</issue_description>

<agent_instructions>fix this and put a strong emphasis on clean documentaiton</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes error when using tutorial for existing instruments #6

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