check packages on init

[piotrm0]

Added checking for installed packages to main __init__ so we can catch problems before hard-to-debug errors arise.

If a user does not have a required package installed, an error is raised:

ModuleNotFoundError: 
Required package langchain is not installed. Please install it with pip:

    ```bash
    pip install 'langchain>=0.0.354'
    ```

If your distribution is in a bad place beyond this package, you may need to
reinstall trulens_eval so that all of the dependencies get installed:
    
    ```bash
    pip uninstall -y trulens_eval
    pip install trulens_eval
    ```

If a user has a required package installed but it is of a wrong version, this error is raised:

VersionConflict: Package langchain is installed but has a version conflict:
	(langchain 0.0.353 (/opt/homebrew/Caskroom/miniconda/base/envs/py311_trulens/lib/python3.11/site-packages), Requirement.parse('langchain>=0.0.354'))

This package is required for trulens_eval. Please resolve the conflict by
installing a compatible version with:

    ```bash
    pip install 'langchain>=0.0.354'
    ```

If you are running trulens_eval in a notebook, you may need to restart the
kernel after resolving the conflict. If your distribution is in a bad place
beyond this package, you may need to reinstall trulens_eval so that all of the
dependencies get installed and hopefully corrected:
    
    ```bash
    pip uninstall -y trulens_eval
    pip install trulens_eval
    ```

If a user has an optional package with a version conflict, a warning is printed:

Package llama-index is installed but has a version conflict:
	(llama-index 0.10.9 (/opt/homebrew/Caskroom/miniconda/base/envs/py311_trulens/lib/python3.11/site-packages), Requirement.parse('llama-index<0.10.0,>=0.9.26'))

This package is optional for trulens_eval so this may not be a problem but if
you need to use the related optional features and find there are errors, you
will need to resolve the conflict:

    ```bash
    pip install 'llama-index<0.10.0,>=0.9.26'
    ```

If you are running trulens_eval in a notebook, you may need to restart the
kernel after resolving the conflict. If your distribution is in a bad place
beyond this package, you may need to reinstall trulens_eval so that all of the
dependencies get installed and hopefully corrected:
    
    ```bash
    pip uninstall -y trulens_eval
    pip install trulens_eval
    ```

If a user does not have an optional package, only debug-level message is printed (not visible by default):

Optional package llama-index is not installed. Related optional functionality will not be
available.