Remove need for parentheses around arithmetic operators

[sbliven]

Currently parentheses are required for all arithmetic operations. This is unintuitive coming from most languages and makes equations more difficult to read. For example:

| I   | II  | III | iv  | sum |
| --- | --- | --- | --- | --- |
| 2   | 3   | 5   | 6   | 5   |
| 3   | 4   | 6   | 7   | 7   |
<!-- TBLFM: $5=($1+($2+($3+$4))) -->

(suggested solution for #70)

There are a number of issues where users struggled with this syntax (#62, #54, #43).

Could the parser syntax be changed to remove this requirement?