Fixes #1245 and fixes #1336 and fixes #1290

The old isolines

• ❌ had a confusing n parameter that tried to pick n isoline values between the min and max,
• ❌ didn't have a way of selecting isoline segments by value
• ❌ tried to numerically merge vertices
• ❌ segments were in arbitrary orientation

This new one

• 😎 create topologically valid isoline polylines
• 😎 takes isovalues as input directly
• 😎 outputs a field indicating which segments belong to which value
• 😎 segments are consistently oriented
• 😎 is a bit fancier than a run of the mill marching triangles: it takes special care to handle constant regions of the scalar field, mesh boundaries, and lines that pass exactly through vertices.

It's reasonably fast. Could be improved but leaving that for another day. For now at least the edge data-structures can be precomputed.