salinity psu/ppt to EC conversion

[dwr-psandhu]

Need this unit conversion function. Does it go here or in pydelmod?

[kjnam]

I vote for VTools, considering it is not only for the Delta, unless we want Vtools to be only for time series.

[water-e]

I would like to avoid a single point of dependence of pyschism on pydelmod. Right now this conversion is in unit_conversions in pyschism. One possibility would be to move the conversions to vtools, but also migrate them to wrap cfunits/udunits at the same time. Basically we would use those tools, but register any new converters and make sure the plumbing works for pandas/xarray.

[water-e]

OK, digging around it seems like cf_units is currently the healthiest implementation of that library.