beautify run without parameters

[obo]

Running SLTev without parameters should emit the help message.

Currently, we're getting this:

(your-env) 14:34 victoria SLTev$SLTev-scripts/SLTev 
Traceback (most recent call last):
  File "SLTev-scripts/SLTev", line 103, in <module>
    if working_dir[-1] != '/':
TypeError: 'NoneType' object is not subscriptable

I would like to see this:

usage: SLTev [-h] [-g [G]] [--simple [SIMPLE]] [--outdir OUTDIR] [-e E]
             [--commitid COMMITID]

generate and evaluate slt/mt/asr files.

optional arguments:
  -h, --help           show this help message and exit
  -g [G]               eliter index name for download files
  --simple [SIMPLE]    if exist just some of the results will be shown
  --outdir OUTDIR      slt output results directory (output of the SLTev)
  -e E                 eliter working directory
  --commitid COMMITID  commitid for eliter-testset repo)

[mohammad2928]

It's done.