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