spacing_symbols may actually contain exprs rather than symbols

[FabioLuporini]

this map may actually be an {expr: float} map rather than a {symbol: float} map. Reproducer is test_dimension.py::TestConditionalDimension::test_subsampled_fd. The ConditionalDimension's spacing is a Mul between its factor and the parent's spacing.

This inconsistency forced me to add an AttributeException here and raise a warning

I did try to fix it, but after 10 mins I was still unsure about what to do, maybe it's obvious to you @mloubout or we can talk about it

[FabioLuporini]

@mloubout actually, is it possible we have fixed this recently? mhh