Generate Confusion matrix related to spectral types

[profxj]

Compares truth vs type recovered from RedRock.

Generates a dict holding the matrix.

See https://github.com/desihub/desisim/blob/confuse_me/doc/nb/Confusion_matrix_spectypes.ipynb
for usage.

Have tested on 18.3 mini-test with:

from astropy.table import Table
from desisim.spec_qa import redshifts as dsqa_z

simz_tab = Table.read('truth.fits')
zcat = Table.read('zcatalog-mini.fits')

results = dsqa_z.spectype_confusion(simz_tab, zb_tab=zcat)

Addresses Issue #348