R functions to filter a vcf

These functions can be used to filter a vcf file. They can remove loci that have a given amount missing data, remove individuals that have a given amount of missing data, remove individuals by name, and create histograms either of missing data per locus or individuals. Finally, once data are filtered, there is a function to write a structure output file.本系列工具可用于对变体调用格式（Variant Call Format，VCF）文件进行过滤处理。其可移除存在指定缺失数据量的遗传位点、存在指定缺失数据量的样本，还可按名称移除指定样本，以及针对单个遗传位点或单个样本的缺失数据情况绘制直方图。最终，在完成数据过滤后，可生成Structure格式输出文件（Structure）。