Zarr conversion to int warning

[briangow]

As mentioned in: #114 when trying to convert a nan to an int a RuntimeWarning: invalid value encountered in cast warning is given.

This line is setting a default int value (-32768) for NaNs but was previously trying to do the conversion to an int prior to
changing the NaNs to the default value:

np.where(np.isnan(chunk['samples']), nanval, np.round(chunk['samples'] * chunk['gain']).astype(np.int16))

This PR updates the code to finish the conversion to the default value prior to converting to an int which avoids the warning message.

[del42]

@briangow please merge! Thank you.