Move automatic conversion from `write` to `encode`

[targos]

I think each encode*** function should handle its own automatic conversion based on what the encoder supports, or throw if the conversion cannot be done in a meaningful way.

[stropitek]

It would be helpful to have a table in the documentation that shows how each color model is encoded

Have on column with the color model / bit depth of the original image and another with the color model / bit depth of decode(encode(image))

[EscapedGibbon]

Here is what I came up with.

DECODE	8bits	16bits	32bits	Gray	RGB	Alpha
TIFF	Yes	Yes	Yes	Yes	Yes	Yes
PNG	Yes	Yes	No	Yes	Yes	Yes
JPEG	Yes	No	No	Yes	Yes	No

BMP is not there because it is not supported for decoding.TIFF and PNG can be decoded as anything. JPEG converts gray and rgb 8-bit images, but once decoded it sets colorModel as RGBA.

ENCODE	1bit	8bit	16bit	Gray	RGB	Alpha
PNG	No	Yes	Yes	Yes	Yes	Yes
JPEG	No	Yes	No	No	Yes	No
BMP	Yes	No	No	No	No	No

We don't support TIFF encoding. JPEG converts any image into 8 bit RGB. Again, important to note that it works only if the data's colorModel is set as RGBA. PNG has the same encoding capabilities as decoding. BMP can encode only binary images.