Add database network check to DbParams

[volcan01010]

Summary

Adding a is_reachable() method to DbParams would allow users to easily check if they can reach their target database over the network. This can be helpful in debugging situations.

Details

The method could be used as follows:

if not my_db.is_reachable():
    raise ETLHelperError("network problems")

It would not require any other parameters and can use Python's socket feature just to see if it is possible to connect to the required host and port. There is already a simple implementation of this in the testing code of etlhelper:

https://github.com/BritishGeologicalSurvey/etlhelper/blob/a18bb66fc319c7d8fd4bc676bdf56f4807d5b92d/test/conftest.py#L119

Acceptance criteria

• [x] is_reachable() function added to DbParams class
• [ ] conftest.py uses new method
• [x] README.md describes how to use.

[volcan01010]

@ximenesuk - if we add this, it will help with future debugging.

[volcan01010]

Closed by #33