The program 'subseq_fr' receives as a parameter from the command line the input fasta file,
the length of the desired subsequence, and the genome strand ('f' for forward, and 'r' for
reverse).

The input file for this program is assumed to be the output of the 'formater' program, it 
makes that all the secuence names are correlatives numbers, these numbers will be included 
in the output sequence's names.

The output of this program is given in the standard output, so if you want to use an output 
file you need to redirect the output to a file (in linux you can do this with the '>' symbol).

The output consists of a series of sequences, each one of them is a subsequence of the 
original, whose names are structured as follows:

> <Sequence Number> <Sub-Sequence First Position> <Sub-Sequence Last Position>

IMPORTANT: The correlative numbers of the sequences and the positions start in 0. The numbering
of subsequence positions is always in reference to the forward strand (i.e. the 5' position
of a reverse subsequence will be always larger than the last position).

If a secuence in the input file has a length that is smaller than the requested as subsequence 
length, it will not be any subsequence of this in the output, i.e. if we have a sequence of 23 
nucleotides and it is asked for subsequences of length 25, this sequence will not have
subsequences in the output (this could cause that some correlative numbers in the output list 
are missing because the input name is preserved).

This program is open source, and thus every program based on this should be open source as well.