qmc-fit doesn't have a clean help option

[ye-luo]

Describe the bug
-h

$ qmc-fit -h

  error:
    unknown fit type: -h
    valid options are: ['ts', 'u']
  exiting.

  File "/home/yeluo/opt/qmcpack/nexus/bin/qmc-fit", line 559, in <module>
    error('unknown fit type: {0}\nvalid options are: {1}'.format(fit_type,fit_types))
  File "/home/yeluo/opt/qmcpack/nexus/lib/generic.py", line 138, in error
    traceback.print_stack()

--help

$ qmc-fit --help

  error:
    unknown fit type: --help
    valid options are: ['ts', 'u']
  exiting.

  File "/home/yeluo/opt/qmcpack/nexus/bin/qmc-fit", line 559, in <module>
    error('unknown fit type: {0}\nvalid options are: {1}'.format(fit_type,fit_types))
  File "/home/yeluo/opt/qmcpack/nexus/lib/generic.py", line 138, in error
    traceback.print_stack()

It would be better to have a nice help output

[prckent]

Good point

--help should be implemented for consistency with qmca

Since qmc-fit uses OptionParser like qmca, adding --help should be quick.