Create an option to control the exponent base instead of subsuming this in exp_mode.

[jagerber48]

Right now users can switch to base 2 exponents by using exp_mode="binary" or exp_mode="binary_iec". This is an idea to introduce a new option like base that can at least take on the values 10 and 2. The "binary" and "binary_iec" modes could then be eliminated and subsumed into the other modes, paired with base=2.

One could imagine generalizing the formatting to arbitrary bases. However, base 2 is already barely supported and it's not clear there is much demand for base 2 formatting, so such an enhancement is a long ways off.