shorten the desi_proc slurm and log filenames using new name convention

[akremin]

Shorten filename of slurm scripts and log files for desi_proc using create_camword. Corresponding function decrypt_camword can convert back to a list of cameras/spectrographs.

create_camword(cameras):

    Function that takes in a list of cameras and creates a succinct listing                                                      
    of all spectrographs in the list with cameras. It uses "a" followed by                                                       
    numbers to mean that "all" (b,r,z) cameras are accounted for for those numbers.                                              
    b, r, and z represent the camera of the same name. All trailing numbers                                                      
    represent the spectrographs for which that camera exists in the list.                                                        
                                                                                                                                 
    Args:                                                                                                                        
       cameras (1-d array or list): iterable containing strings of                                                               
                                     cameras, e.g. 'b0','r1',...                                                                 
    Returns (str):                                                                                                               
       A string representing all information about the spectrographs/cameras                                                     
       given in the input iterable, e.g. a01234678b59z9   

decode_camword(camword):

    Function that takes in a succinct listing                                                                                    
    of all spectrographs and outputs a 1-d numpy array with a list of all                                                        
    spectrograph/camera pairs. It uses "a" followed by                                                                           
    numbers to mean that "all" (b,r,z) cameras are accounted for for those numbers.                                             
    b, r, and z represent the camera of the same name. All trailing numbers                                                     
    represent the spectrographs for which that camera exists in the list.                                                       
                                                                                                                                
    Args:                                                                                                                        
       camword (str): A string representing all information about the spectrographs/cameras           e.g. a01234678b59z9 
                   
    Returns (np.ndarray, 1d):  an array containing strings of cameras     e.g. 'b0','r1',...        

[sbailey]

This looks good; I tried a bunch of combinations and wasn't able to break it, and confirmed that round trips create_camword -> decode_camword work.

Partially for practice, please add some unit tests to py/desispec/test/test_io.py to test that various combinations produce the expected results with missing cameras, full b/r/z sets, round trips of create_camword -> decode_camword -> create_camword work, etc.

[sbailey]

Looks good, thanks. Will merge once Travis finishes.