Extend BC-S1.2 format to support variables projection

[ecivini]

This PR extends the BC-S1.2 format and parser to support the specification of projection variables. It adds a new optional statement P v1 v2 ... vn to the format, where vx are the projection variables.

Considering the compilation case, a new example would be:

c BC-S1.2
I a
I b
P a
G g1 := O a b
G g2 := O -b a
G g3 := A g1 g2
T g3

which produces the following output:

o 1 0
t 2 0
1 2 1 0

Without the projection, the output is the following:

o 1 0
t 2 0
1 2 1 2 4 5 0