GH-464 Add log function to SPARQL

[ate47]

Issue resolved (if any): #464
Description of this pull request:

Add the log function inside the SPARQL endpoint. It can be used with <http://the-qa-company.com/qendpoint/#log>(value, base = 10). For example:

PREFIX qep: <http://the-qa-company.com/qendpoint/#>

SELECT * {
	VALUES ?number { 10 80 420 10000 20000 1000000 }
	BIND (qep:log(?number) AS ?logNumberDef)
	BIND (qep:log(?number, 10) AS ?logNumber10)
	BIND (qep:log(?number, 16) AS ?logNumber16)
	BIND (qep:log(?number, 2) AS ?logNumber2)
}

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