implement direct variable queries & local scopes

an enhancement towards more convenient working with variables in Octave's current scope

summary of changes:
setvar, getvar and isvar are implemented to set, get and test existence of variables.

this allows doing things like

pytave.setvar("x", 1)
pytave.eval(0,"x += 1")
x = pytave.getvar("x")

push_scope and pop_scope are provided to create an anonymous scope and push it on Octave's call stack,
to prevent cluttering other variables if nested calls to pytave are in effect.

pytave.push_scope()
pytave.setvar("x", something)
pytave.eval(0, "... do something with x)
pytave.pop_scope() # wipes out x and restores its previous value, if any

of course, for proper safety, try/finally block should be used to ensure the cleanup. for convenience, a local_scope decorator is provided that encloses a whole function in push_scope/try/finally/pop_scope sequence:

@pytave.local_scope
def my_oct_add(x,y):
    pytave.setvar("x",x)
    pytave.setvar("y",y)
    result, = pytave.eval(1, "x + y")
    return result

this function, when called, will not affect the top-level values of x and y, if any.

patch attached