Support single channel f32 tiff files

[maxall41]

Adds support for single channel f32 tiff files. See #2251.

[fintelia]

Thanks for your interest!

Sorry for not getting into the details in the linked PR, but we intentionally limit the number of variants of ColorType and DynamicImage because users have to be able to handle getting any possible variant when they decode an image file. On the other hand, the TIFF format allows a nearly unbounded number of pixel formats including {u8, u16, u32, ..., i8, i16, ..., f16, f32, f64, ...} x {1, 2, 3, 4, ...} samples per pixel, so we have to draw a line.

A big factor to guide that decision has been whether the pixels of a given type would generally represent colors. My understanding is that single channel float TIFFs tend to be used for GIS data or other non-color information, and thus better not to include in ColorType / DynamicImage.

[maxall41]

Ah ok Ill just maintain a fork then.