Improve precision

[rubensworks]

New version of https://github.com/comunica/sparqlee/pull/43

Used the library decimal.js to do mathematical operations for us.

Notes:

• Option to avoid Decimal operation has not been implemented yet.
• I may have missed some operations.
• This PR changes the following implementations from the RegularFunctions file: multiplication, division, addition, substraction.
• Could benefit from a more efficient implementation. Maybe using Decimals internally?

Fixes SUM test from comunica/comunica#287

[wschella]

I created #45 for the efficiency remark

[coveralls]

Pull Request Test Coverage Report for Build 241

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 79.609%

---

Totals
Change from base Build 231:	0.01%
Covered Lines:	1046
Relevant Lines:	1263

---

💛 - Coveralls