Add support for Latex printing of models

[jessegrabowski]

Users should be able to label equations and variables in the GCN file somehow. Here is how it could look using Python-style decorators for example:

@name: Law of Motion of Capital
K[] = (1 - delta) * K[1] + I[];

Variables could also be given shorthand representations to prevent descriptive (but long) variable names from getting printed to Latex:

shocks
{
    @shortname: epsilon[]
     epsilon_technology[];
};

Then, of course, the user should be able to call some kind of print_latex_model function to get a nice latex table with all model equations, steady state values, priors, etc.