semantic expansion of units

[d-chambers]

@quasiStellar45 came up with a good idea about how we might use units.

Normally, to indicate values passed to a patch function represent samples, the patch function has a samples argument. Something like this:

import dascore as dc 

patch = dc.get_example_patch()
# trim to only include the first 10 samples on time dimension
sub_patch = patch.select(time=(..., 10), samples=True)

but it might be done like this:

import dascore as dc 
from dascore.units import samples

patch = dc.get_example_patch()
# trim to only include the first 10 samples on time dimension
sub_patch = patch.select(time=(..., 10 * samples))

We also discussed allowing size constraints to be specified by memory units. For example:

from dascore.units import GB

# specify patches created from chunk should have 2GB of array data
chunked_spool = spool.chunk(time=2*GB)

This would require storing the array dtype in the index (which we don't current do), and possibly tie into fixing #252.