iFit/multifit

 Simultaneously fit a function to an array of objects.
 Optionally allows background functions that vary independently for each object. 

 Synonymous with multifit_func. For full help, read documentation for multifit_func:
   >> help sqw/multifit_func

 Simultaneously fit several objects to a given function:
   >> [wout, fitdata] = multifit (w, func, pin)                 % all parameters free
   >> [wout, fitdata] = multifit (w, func, pin, pfree)          % selected parameters free to fit
   >> [wout, fitdata] = multifit (w, func, pin, pfree, pbind)   % binding of various parameters in fixed ratios

 With optional 'background' functions added to the global function, one per object
   >> [wout, fitdata] = multifit (..., bkdfunc, bpin)
   >> [wout, fitdata] = multifit (..., bkdfunc, bpin, bpfree)
   >> [wout, fitdata] = multifit (..., bkdfunc, bpin, bpfree, bpbind)

 If unable to fit, then the program will halt and display an error message. 
 To return if unable to fit, call with additional arguments that return status and error message:

   >> [wout, fitdata, ok, mess] = multifit_sqw (...)

 Additional keywords controlling which ranges to keep, remove from objects, control fitting algorithm etc.
   >> [wout, fitdata] = multifit (..., keyword, value, ...)

   Keywords are:
       'keep'      range of x values to keep
       'remove'    range of x values to remove
       'mask'      logical mask array (true for those points to keep)
       'select'    if present, calculate output function only at the points retained for fitting
       'list'      indicates verbosity of output during fitting
       'fit'       alter convergence critera for the fit etc.
       'evaluate'  evaluate function at initial parameter values only, with argument check as well
       'chisqr'    evaluate chi-squared at the initial parameter values (ignored if 'evaluate' not set)

   Example:
   >> [wout, fitdata] = multifit (..., 'keep', xkeep, 'list', 0)