Add additional program mode: Filter

[mvences]

This is not really a priority, but I am writing it down here as it came to my mind today. If it is easy to address it can as well be implemented quickly.

After today's work with different sequence formats, I realized that it really would be great to have one more program mode for the program which we could call FILTER. I will create a new issue on Githib for that. Basically it would be about manipulating and filtering sequences. For now it would be enough to include the following filters (some of them are indeed included in FASTMERGE and FASTSPLIT, so perhaps Vladimir can easily add them to the backend ... or maybe they are so trivial that Stefanos can just quickly program them himself:

• Remove sequences that are below or above a certain length
• Remove sequences that contain a certain search term in the sequence name (and alternatively, remove sequences that do NOT contain the search term)
• Remove sequences that contain a certain sequence motif (something like "AAACGCAA") in their sequence (and alternatively, remove sequences that do NOT contain this motif).
• Perform simple search/replace operations either only in the sequences, or only in the sequence names.