added a dm_ism script

[lordrick94]

Added a script that can

1. Create a dm_ism healpix map
2. Function to grab dm_ism for given l,b (much faster than calculating over and over again from ne2001 model)
3. Get option to see metadata for the generated mapfile on disk(repo)
4. Plot option to plot the file on disk(repo)

Also added a mapfile previously generated. Path: frb/dm_ism_models/ne2001_dm_healpix_map.fits

[profxj]

Thanks for this!

Need to add a test or two and also
a bit of docs.

[profxj]

all set!