Update expanding() function in algebra.py

[ialoko]

Updated algebra/expanding string integer out of range error

Fixes #403 I fixed the bug causing the string out of range error .

I also fixed some other issues where the function was allowing invalid algebraic expressions.

I also fixed the output so that it takes into account situations where a number is zero for either or all terms.

Fixed some other minor formatting issues. Tested my function and works correctly.