Changes to the DB reading method.

examples/script_design_capacity.py

examples/script_moody_diagram.py

examples/script_nikuradse_experiments.py

examples/script_pipe_losses.py

src/topupheat/pipes/utils.py

tests/test_all.py

tests/test_pipes.py

tests/test_system.py

tests/test_trenches.py

tests/test_utils.py