ENH: Multiprocessing for THUMBNAIL

[pllim]

Enable multiprocessing for downsampling images in THUMBNAIL mode. I checked that output images did not change at all (which is good) using fitsdiff. Also changed the default output dimension from 100 to 500 as @Skyhawk172 suggested.

Some timing:

In [1]: import glob

In [2]: images = glob.glob('*.fits')

In [3]: len(images)
Out[3]: 10

In [4]: from wss_tools.quip.main import  shrink_input_images

In [5]: %time out = shrink_input_images(images, ext=('SCI', 1), new_width=500, outpath='ztmp1', n_cores=1)  # No multiprocessing
CPU times: user 1.38 s, sys: 173 ms, total: 1.55 s
Wall time: 1.58 s

In [6]: %time out = shrink_input_images(images, ext=('SCI', 1), new_width=500, outpath='ztmp2', n_cores=10)
CPU times: user 3.55 ms, sys: 23.4 ms, total: 26.9 ms
Wall time: 297 ms

[pllim]

Need this for #70 , so merging.