Changes in colToProp and propToCol methods

[wszymanski]

Context

This PR realise idea to return null for indexes beyond the table boundaries while calling propToCol method. However, there should be done yet more significant changes which affect lot of places in Handsontable.

• propToCol returns null for not existing properties
• colToProp returns null for indexes beyond the table boundaries
• colToProp return null for not existing property

handsontable/src/dataMap/dataMap.js

Lines 198 to 201 in 398178d

	// TODO: This should be removed. Please keep in mind that the `getSourceDataAtCol` and `getSourceDataAtCell` use it.
	if (Number.isInteger(column) === false) {
	return column;
	}

• should colToProp return -1 after handling -1 as argument (considering row header)?
  • colToProp return null after handling -2 as argument (considering row header)?
• should colToProp and propToCol work on physical indexes (to perform translation also on trimmed indexes properly)?
  • should colToProp(number) and propToCol(number) return number or translated number (to visual/physical index)?
• should colToProp(x) === propToCol(colToProp(x))?
• methods such as getSourceDataAtCol, getSourceDataAtCell and getAtCell (internally function) work on physical indexes.

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