Add additional checking for existence of passed reference genome

[apetkau]

Description of feature

It is possible to pass a non-existent path to the --refgenome parameter used to pass a reference genome into a pipeline. The Nextflow JSON Schema includes a keyword, exists, which can be used to check if a passed file exists on validation of parameters: https://nextflow-io.github.io/nf-validation/nextflow_schema/nextflow_schema_specification/#exists. This keyword is not used for the --refgenome parameter:

snvphylnfc/nextflow_schema.json

Lines 53 to 59 in adafa83

	"refgenome": {
	"type": "string",
	"description": "The file path to the FASTA-format reference genome.",
	"fa_icon": "fas fa-file",
	"pattern": "^\\S+\\.(fa|fas|fasta)$",
	"format": "file-path"
	}

This request is to add the exists keyword to this parameter.

Note that the pipeline will still fail if an invalid file is passed, it will just fail at a later stage of the pipeline (tested in Azure).