Unit of position doesn't return from aperture_photometry for singe positions #212
Description
This should be fix before 0.1
The units of pixel gets forgotten along the way if the position has only one element:
>>> pos=SkyCoord(0, 0, unit=(u.deg, u.deg))
>>> aperture_photometry(data, SkyCircularAperture(pos, 1*u.arcsec))
<Table rows=1 names=('aperture_sum','xcenter','ycenter','center_input') units=('MJy / sr',None,None,None)>
array([ (2.949068297096934e-06, [99.0], [49.0], <SkyCoord (ICRS): ra=0.0 deg, dec=0.0 deg>)],
dtype=[('aperture_sum', '<f8'), ('xcenter', '<f8', (1,)), ('ycenter', '<f8', (1,)), ('center_input', 'O')])
while it's working well for multiple positions...
>>> pos=SkyCoord([0, 0], [0, 0], unit=(u.deg, u.deg))
>>> aperture_photometry(data, SkyCircularAperture(pos, 1*u.arcsec))
<Table rows=2 names=('aperture_sum','xcenter','ycenter','center_input') units=('MJy / sr','pix','pix',None)>
array([ (2.949068297096934e-06, 99.0, 49.0, <SkyCoord (ICRS): ra=0.0 deg, dec=0.0 deg>),
(2.949068297096934e-06, 99.0, 49.0, <SkyCoord (ICRS): ra=0.0 deg, dec=0.0 deg>)],
dtype=[('aperture_sum', '<f8'), ('xcenter', '<f8'), ('ycenter', '<f8'), ('center_input', 'O')])
@cdeil - this can be assigned to me