Support multi-sample VCF files as input

[sci-kai]

This PR addresses Issue #35 for supporting multi-sample VCF files.

The following changes were made:

• removed the check for single_samples in vcf_check.py
• changed create_vembrane_fields.py to automatically parse samples in vcf. This makes use of the "for_each_sample()" function implemented in vembrane. All fields specified in the parameters that affect the FORMAT fields are done for each sample in teh vcf and included into the reports with the VCF-internal sample name incorporated into the column name.
• Changed documentation accordingly and fixed minor formatting and spelling errors.

[sci-kai]

I investigated this issue and it seems to be related with vembrane handling missing entries in the FORMAT field. This is discussed in https://github.com/vembrane/vembrane/issues/171 Hence multi-sample VCF files only work if they do not have missing entries. (e.g., if mutect2 calls tumor-normal pairs this is the case, but merged VCF files from different callers may not work). This should be changed within vembrane to handle missing entries.