take rate and duration as commandline parameters

The duration is described in 30 minute intervals.
Eg, a duration of 0x01 is 30 minutes.

The rate is described in 0.025 increments, at least on a 515.
Eg, a rate of 0x01 is 0.025U/H.

Conversion for both goes like this:
+  duration = args.duration / 30
+  rate = int(args.rate / 0.025)

The temp basal command, is 76, eg 0x4C hex, and takes a parameters
list of 3 bytes.

I have no idea what the first byte does, this particular logic only
works when the first byte is set to 0.

This logic sets an absolute rate of units/hour, not a percentage,
there may or may not be another way to do that.

The second byte of the parameters represents the rate, and the third
is the duration.

The rate is an integer describing 30 minute increments.  Duration is
an integer describing 0.025 unit increments.

See --help for more on the --rate and --duration parameters, which
takes minutes, and absolute units per hour.
make sure --rate allows floats