Error from api.clean.find_surface_from_P(ds, salinity=XX)

[calumkenny]

Describe the bug:

Error experienced when running api.clean.find_surface_from_P(ds, salinity=XX) function:

AttributeError: 'NoneType' object has no attribute 'pressure'

To Reproduce:

Use function on RDI ADCP without pressure variable.

Expected behavior:

Error occurs.

Additional context:

Working session with @jmcvey3.

[akeeste]

Resolved with #354