Add config files for extended_k (k bond value changing with swimmer length)
-> Changes on addswimmer
   - add new bond_extended parameter ( = 0 -> harmonic/swimmer
     	     		   	       = 1 -> harmonic/swimmer/extended
                                       = 2 -> harmonic/swimmer/extended/k)
-> Add new k_alpha and k_beta parameter in "in.swimmer"
-> Longer swimmer_length (150)