model.py: Record class: changes in merge function and add_call function added. Reader class: merge method added. Method merge_info_from_vcf_files added in parser.py
parser.py: Writer: minor changes for errors output
model.py: Record class: changes in merge function and add_call function added. Reader class: merge method added. Method merge_info_from_vcf_files added in parser.py
parser.py: Writer class: add_attr method created, for adding infos, formats, etc. _init_: now does not write header until first record will be written, since further information may be added
model.py: Record class: get_alternative method added: Returns a new record containing all the information of current record but only for the
        alternative with the selected index
model.py: Call class: af property: VarScan calls include only one allele freq value, that is assumed for all alternative alleles in case of 0/0 genotypes