finds default coefficients in equations
coef=defaultcoef(defaultname,speccarb,speccara,arg1,...,argn)
* defaultname = a string (default prefix of the coefficients)
* speccarb = the characters that must be before a coefficient to be sure that defaultname is not part of the name of another object
* speccara = the characters that must be after a coefficient to be sure that defaultname is not part of the name of another object
* argi = a string representing the text of an equation
* coef = a (nx1) vector of coefficients names