Use constant in the parse tree for minus + graphviz export handles constants

[arnaud-m]

• close [FEATURE] Use a constant in the parse tree for minus sign  #45
• close [BUG] Remove question marks of constant in graphviz export #48

[FissoreD]

Ok, I've seen your modifs.
And I really like your idea to put 0 before unary minus, because I have an idea to propose you next week about how we can do subtraction with bigNumm without moving subtrees on the other side of the equal.
For the moment my approach with minus has some bugs, but maybe you will find a trick to make it work (if my idea is not completely incohrent).
This week I've also thought about how we can implement also the NE operator with BigNum normally it should work.
For the PR it's good for me.

[arnaud-m]

• I am curious to hear about your idea for handling minus.
• Do not spend to much time on the ne operator. Inequalities will often have multiple solutions and therefore not considered as cryptarithms.