Support 16-bit tiffs

[GretaCB]

Tried uploading a local 16-bit tiff and no good:

@kapadia suggested scaling the pixel values into 8-bit values before displaying, since computer screens can't display all values in a 16-bit image.

[brunosan]

Rescaling is the best option. For background, almost no instrument actually uses the whole 16 bit range. E.g. Landast 8 is 12bits (0-4096) rescaled to 16bit (actually 0-55000, less than 2^16), so when doing gdal_translate -ot Byte -scale 0 55000 0 255 you are ok. Landsat 7 is 8bit but after LEDAPS is 12 bit, ...

The longer answer is that we could safely cap at 10000 (~of 100% reflectivity, nothing that we usually care reflects more than ~80-100%) or even lower depending on the instrument and using the metadata. This is good becuase we keep as much dynamic range as possible.

[GretaCB]

Notes for myself:

• [ ] check node-gdal for gdal_translate implementation
• [ ] Checkout gdal_contrast_stretch
• [ ] account for percentile min/max of 16-bit file (1, 99)