Fixes to transect extraction and plotting

[xylar]

This merge fixes several issues with transects:

• Sometimes, when a segment of a transect ends exactly at an intersection with a cell edge, there are redundant intersection points in the list. This merge removes those redundant points.
• This merge also fixes some index arrays that were previously type float but should be int.
• Finally, this merge fixes masking of the SSH to ignore cells where maxLevelCell==0

[xylar]

Testing

I used this branch along with https://github.com/xylar/polaris/tree/add-isomip-plus-init to produce the following transect plots. Without these fixes, the plot could not be produced. Without the SSH fix, transects include a jump to SSH = 0 in the region of grounded ice.