Simpler API for one-band use-cases

[m-mohr]

I'd suggest making the one-band use case a bit simpler by allowing 1 and 2 elements for the bands option, too.

• 1 element means single band w/o alpha (i.e. expand from e.g. [0] to [0,0,0])
• 2 elements mean single band with alpha (i.e. expand from e.g. [0, 1] to [0,0,0,1])
• 3 and 4 as defined below.

| `bands`                      | array of numbers | undefined | an array mapping the bands to the output bands. must be of length 1, 2, 3 or 4. |

_Originally posted by @m-mohr in https://github.com/stac-utils/stac-layer/pull/50#discussion_r1121370322_

[m-mohr]

Implemented in the stac-js branch