Implement min-avg-qual option

[NumaanFormoli]

• Minimum base quality at a position to count a read
• Check VarScan Doc

[adityaparmarr]

checked documentation here https://varscan.sourceforge.net/using-varscan.html#v2.3_pileup2snp, done implementing. default 15. manually convert between integer and character and probability using Phred score table.