Word size for megablast

[IvanTsers]

Hello Bernhard,

I have added the -W flag to control the Blast word size of the megablast mode in Subtraction 2. The word size specified by the user overrides the task-specific word size, while penalties stay the same. The existing modes don't change. The usage logic:

1. default: megablast followed by blastn:

fur -d db

2. megablast mode:

fur -d db -m

3. megablast with a specific word size:

fur -d db -m -W 18

The test is included.

P.S. Further expansions of Subtraction 2 settings would require some thinking to keep the currently available modes stable. For example, a flag for a custom blast command could be added:

-B "-word_size 18 -gapopen -10 -penalty -1 -culling_limit 2"

Cheers,
Ivan