add an arbitrary-metadata option to parse_variables

[mjburton11]

@bqpd I don't think that parse_variables supports evalfn right now. If there was a syntax something like this, that would be awesome:

class NewModel(Model):
    """
    Variables
    ---------
    x         [-]       stupid var 1
    y         [-]       stupid var 2         evalfn=self.calc_y
    """
    def calc_y(self, v):
        return v(self.x) + 1
    def setup(self):
        exec parse_variables(NewModel.__doc__)

Possible solution is to add this to docstring.py

                while line[labelstart] == " ":
                    labelstart += 1
                label = line[labelstart:].replace("'", "\\'")
                if 'evalfn=' in label:
                    evalfn = label.split('evalfn=')[1]
                    label = label.split('evalfn=')[0]
                else:
                    evalfn = None
            countstr += variable_declaration(nameval, units, label, line, evalfn,
                                             errorcatch)
        if declaration_func is None:
            overallstr += countstr
        else:
            overallstr += declaration_func(string, flag, idx2, countstr)
        string = string[idx2+len(flag):]
    return overallstr


PARSETIP = ("Is this line following the format `Name (optional Value) [Units]"
            " (Optional Description)` without any whitespace in the Name or"
            " Value fields?")


def variable_declaration(nameval, units, label, line, evalfn, errorcatch=True):
    "Turns parsed output into a Variable declaration"
    if len(nameval) > 2:
        raise ValueError("while parsing the line '%s', additional fields"
                         " (separated by whitespace) were found between Value"
                         " '%s' and the Units `%s`. %s"
                         % (line, nameval[1], units, PARSETIP))
    elif len(nameval) == 2:
        out = ("{0} = self.{0} = Variable('{0}', {1}, '{2}', '{3}')")
        out = out.format(nameval[0], nameval[1], units, label)
    elif len(nameval) == 1:
        out = ("{0} = self.{0} = Variable('{0}', '{1}', '{2}', evalfn={3})")
        out = out.format(nameval[0], units, label, evalfn)
    if errorcatch:
        out = """

[bqpd]

Oh interesting. I was thinking of having the syntax be something more like:

class NewModel(Model):
    """
    Variables
    ---------
    x         [-]       stupid var 1
    y         [-]       stupid var 2


    evalfn (variable metadata)
    --------------------------
    y        self.calc_y
    """
    def calc_y(self, v):
        return v(self.x) + 1
    def setup(self):
        exec parse_variables(NewModel.__doc__)

@mjburton11 Would this work for you? It'd be easier to make more general, and I think it works well for specifying robustness parameters, etc.

[pgkirsch]

+1 @mjburton11 for creating this issue, and +1 @bqpd for the proposed syntax.