maf file format, when is given as paremeter, does not follows the plink .frq format

[rcanovas]

In line 754 of randompca.cpp. The code uses read_text to read maf file assuming that the maf file follows the format "SNP Allele MAF". Meanwhile the current .frq format returned by plink (option --freq) follows the format "CHR SNP A1 A2 MAF NCHROBS"