PSSC variable not compatible with VM plots or subsets over a time range

[JustinElms]

Describe the bug
Users cannot produce virtual mooring plots or time-ranged subsets of the Potential Subsurface Channel variable. It appears that the functions used to calculate this variable do not account for input variables with a time dimension which raises a number of errors throughout the PSSC calculation.

To Reproduce
VM issue:

• Select PSSC from the variable dropdown menu.
• Create a point (any method) then click plot.
• Select the Virtual mooring tab.
• The plot query will fail.

Subset issue:

• Draw an area using any dataset that has a PSSC variable (again any method).
• Select PSSC from the variable select box in the subset panel.
• Click the Select Time Range check box and choose any values.
• Click Save and the query will fail.

Expected behavior
The above workflows should function without issue.