feat(Collector): Add support to have alternative import column names

[Hipska]

This solves issues when attribute code does not match import column name or if an attribute has multiple columns.

Custom collectors can now override HeaderIsAllowed method to specify if the column should be allowed or not.

[Hipska]

(not really a bug, more like a feature IMHO)

@piRGoif if the corresponding PR Combodo/iTop#541 would not be accepted, we can reduce this PR to only the addition of HeaderIsAllowed with the fallback method:

	protected function HeaderIsAllowed($sHeader)
	{
		return array_key_exists($sHeader, $this->aFields);
	}

[Hipska]

Can I have any feedback please?

[piRGoif]

Hello,
Any PR submitted has to be handled by a Combodo employee first, who will ensure she/he understand everything and the code is up to our standards. Then the PR will be sent in reviews.
I'm not quite an expert on collectors so I won't handle this PR. I'll ask other Combodo devs that are involved in collectors...

[Hipska]

Okay, thanks. I was mentioning you specifically since you added the bug label, while this is actually a new feature enhancement.

[piRGoif]

I do some triage but not always very at ease on some subjects O:)
I'll change the label

[Hipska]

@Molkobain do you have some (personal?) feedback?

[Molkobain]

Not yet unfortunately, as I'm very not familiar with the collectors, I don't know how to test this before / after without a detailled reproduction procedure and test cases.

I know the PR template is a bit boring, but it's helps me a lot understanding and setuping the PR to handle it. Otherwise it demands me to spend a few hours to dig into it which I don't have often. :/