Closes #2878: Add groupby for floats

[stress-tess]

This PR (closes #2878) adds groupby for floats by reinterpretting their bits as a uint (using transmute) and using the resulting uint pdarray for the groupby

Verification that groupby and related methods work on floats as expected

>>> select_from =  ak.linspace(2**65, 2**66, 10)
>>> idx = ak.randint(1, 10, 100)
>>> group_on = select_from[idx]
>>> ak.GroupBy(group_on).count()
(array([4.099276460824345e+19 4.5092041069067796e+19 4.9191317529892135e+19 5.3290593990716482e+19 5.7389870451540828e+19 6.1489146912365167e+19 6.5588423373189513e+19 6.968769983401386e+19 73786976294838206464.00000000000000000]),
 array([9 10 14 10 11 6 17 11 12]))

>>> ak.GroupBy([group_on,group_on]).count()
((array([4.099276460824345e+19 4.5092041069067796e+19 4.9191317529892135e+19 5.3290593990716482e+19 5.7389870451540828e+19 6.1489146912365167e+19 6.5588423373189513e+19 6.968769983401386e+19 73786976294838206464.00000000000000000]),
  array([4.099276460824345e+19 4.5092041069067796e+19 4.9191317529892135e+19 5.3290593990716482e+19 5.7389870451540828e+19 6.1489146912365167e+19 6.5588423373189513e+19 6.968769983401386e+19 73786976294838206464.00000000000000000])),
 array([9 10 14 10 11 6 17 11 12]))

>>> ak.sort(group_on)
array([4.099276460824345e+19 4.099276460824345e+19 4.099276460824345e+19 ... 73786976294838206464.00000000000000000 73786976294838206464.00000000000000000 73786976294838206464.00000000000000000])

>>> ak.argsort(group_on)
array([2 14 16 ... 42 67 71])

>>> ak.coargsort([group_on,group_on])
array([2 14 16 ... 42 67 71])

>>> ak.unique(group_on)
array([4.099276460824345e+19 4.5092041069067796e+19 4.9191317529892135e+19 5.3290593990716482e+19 5.7389870451540828e+19 6.1489146912365167e+19 6.5588423373189513e+19 6.968769983401386e+19 73786976294838206464.00000000000000000])

>>> ak.GroupBy(group_on).unique_keys
array([4.099276460824345e+19 4.5092041069067796e+19 4.9191317529892135e+19 5.3290593990716482e+19 5.7389870451540828e+19 6.1489146912365167e+19 6.5588423373189513e+19 6.968769983401386e+19 73786976294838206464.00000000000000000])