[Bug]: Arithmetic operations on multiple-coordinates fails

[fernandezc]

What happened?

for multi-coordinates, the following simple operation fails:

dataset.y -= dataset.y[0]

This is because the slicing of the multi coordinates do not return a CoordSet object but a Coord.

A workaround is to explicitly set which of the sub-coordinate is used:

dataset.y_1 -= dataset.y_1[0]

What did you expect to happen?

The operation should act transparently on the default coordinate.