Let correlation endpoint accept only one dataset

[asizemore]

Resolves #92

For wgcna, we'd like the user to run correlation on 1 (self-correlation) or 2 datasets. Currently we have a separate endpoint for self-correlation.

This PR does a few things:

1. Renames hasSecondCollection to hasTwoCollections
2. Removes restrictions on data2 being null and data2 == data1
3. Allows for self-correlation calculations to be performed by either passing data1=data2 or data1 and data2=null.

It's looking like the frontend will be sending in data2==data1. Still, both this avenue and data2=null work and seem like reasonable ways to use the endpoint, so i'm partial to keeping both options but could be persuaded otherwise!

To test, use eda-inc and here's an example request:

{
	"config": {
		"prefilterThresholds": {
			"proportionNonZero": 0.05,
			"variance": 0,
			"standardDeviation": 0
		},
		"data1": {
			"dataType": "collection",
			"collectionSpec": {
				"entityId": "EUPATH_0000813",
				"collectionId": "EUPATH_0009252"
			}
		},
		"correlationMethod": "spearman"
	},
	"derivedVariables": [],
	"filters": [],
	"studyId": "Bangladesh_healthy_5yr-1"
}

To test data1 == data2,

{
	"config": {
		"prefilterThresholds": {
			"proportionNonZero": 0.05,
			"variance": 0,
			"standardDeviation": 0
		},
		"data1": {
			"dataType": "collection",
			"collectionSpec": {
				"entityId": "EUPATH_0000813",
				"collectionId": "EUPATH_0009252"
			}
		},
		"data2": {
			"dataType": "collection",
			"collectionSpec": {
				"entityId": "EUPATH_0000813",
				"collectionId": "EUPATH_0009252"
			}
		},
		"correlationMethod": "spearman"
	},
	"derivedVariables": [],
	"filters": [],
	"studyId": "Bangladesh_healthy_5yr-1"
}

[asizemore]

@bobular if possible i'd like to merge this quickly and come back to fix things in a new PR if possible. I'm trying to work on the frontend simultaneously but running the backend locally is slowing dev.
Since we'll be testing with local frontend dev in the next week, i dont think extensive testing is required.

[asizemore]

Don't review! I've introduced a bug and i must squash it first. Can see that now mbio correlations don't work