[core] Ensure we cast sample rate to a float

[brettlangdon]

If a string is passed in then we get the following error:

>>> '1' * (2 ** 64)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OverflowError: cannot fit 'int' into an index-sized integer

Explicitly cast sample rate to a float to at least raise a TypeError if it isn't "floatable"