Syntax improvements and additions

[mitchphillipson]

Endowments

In MPSGE there is syntax of the form

e: E   q:Q    r:RATE

This is equivalent to

e: E    q:Q*RATE

We should add a keyword to the endowment constructor of the form

@endowment(E, Q, ration= RATE)

[mitchphillipson]

Taxes

It's possible in a netput to have something of the form

I:X    q:Q    A:CONS    N:Z     M:C

This is equivalent to

I:X    q:Q    A:CONS    t:Z*C

N stands for endogenous and M for multplier

[mitchphillipson]

Productivity Externality

Currently in MPSGE we must do

i:P   Q: L*x    P: 1/L

It would be preferable to have syntax of the form

i:P   Q: x   X: L