Add command-line arguments

Used argparse package to add command-line arguments for the 4 inputs: FASTA sequences, counts CSV, and mean+std fragment lengths. Included check that FASTA and CSV file paths are valid.

frag_package/fragmentation.py

-import pandas as pd
 import random
-import numpy as np
+
 
 dna_seq = {
     "ATAACATGTGGATGGCCAGTGGTCGGTTGTTACACGCCTACCGCGATGCTGAATGACCCGGACTAGAGTGGCGAAATTTATGGCGTGTGACCCGTTATGC": 100,