indexing with intervals has broken

[ssfrr]

Not sure when this broke, but indexing with intervals seems to be having trouble.

using SampledSignals
using IntervalSets

b = SampleBuf(randn(10), 48000)
b[1..3]

gives ERROR: DimensionError: 1 Hz s and s are not dimensionally compatible.. The problem seems to be in the code that converts to raw indexes, where it's introducing a frame unit and causing problems. Related to #35 but that passed CI so I'm not sure what changed.

[ssfrr]

I think the issue is that Quantity is a more specific type than DimensionlessQuantity. I'm guessing that wasn't the case when #35 was merged, but changed in Unitful.jl at some point. Should be fixed by #53