Add option to decode as unscaled signed integers

[atkinson153]

The 4 bit decoder follows scaling conventions that some users may want to avoid, so the aim is to add an option where instead the signed ints get returned rather than the scaled values.

For now, I deal with it in hacky way (doc entry to be entered here)

[mhvk]

Just as a reminder to ourselves: a possible way we can do this is to have an decoder argument to the standard readers, which allows putting in a function that returns whatever the user wants if given an array of raw data. We could have a number of defaults (perhaps keyed by a string, one for the current default ("mark5access"?) and maybe another for "int" or "unscaled".

The one thing that may need some thought is what exactly the decoder input and output are supposed to be. For input, is it the true raw data, or the data already split in unsigned ints for each sample (e.g.,, one for each 4 bits in CHIME). For the output, is decoder responsible to combining 2 samples into complex as needed? This currently is done separately but it feels more reasonable to leave this to the user.

Of course, if we do give readers a decoder option, we probably should also give writers an encoder one...