Encode float values for PLI images

Currently visualizing PLI images containing float values with Microdraw does not work as expected. Likely a float to integer conversion (accounting for min max values defined by PLI attributes) for the output pngs needs to be implemented inside the [_encode_image](https://jugit.fz-juelich.de/inm-1/bda/software/data_access/tiamat/tiamat-openseadragon/-/blob/master/tiamat_openseadragon/api.py?ref_type=heads#L72) function