Promote floats to durations during arithmetic ops

[anuragm]

oqpy allows adding float literals to durations, but they are not automatically converted to duration literals (unlike initialization of DurationVar or duration arrays, where python floating literals are automatically converted to duration literals).

With this change, literals are automatically promoted. For example,

prog = oqpy.Program()
port = oqpy.PortVar(name="my_port")
frame = oqpy.FrameVar(name="my_frame", port=port, frequency=5e9, phase=0)

prog.delay(delay + 5e-9, frame)
print(prog.to_qasm(encal_declarations=True))

will correctly promote 5e-9 to a duration literal and output

OPENQASM 3.0;
defcalgrammar "openpulse";
cal {
    port my_port;
    frame my_frame = newframe(my_port, 5000000000.0, 0);
}
duration d = 10.0ns;
delay[d + 5.0ns] my_frame;

[PhilReinhold]

Small comments but otherwise looks good