Reduce number of virtual nodes

[martinmoraga]

The use of virtual nodes in the simulation of larger networks is unpractical. For example, the benchmark IEEE14 has only 14 buses, however, in DPsim the dimension of the admittance matrix is equal to 38x38 (and 76x76 in SP/DP domain!):

• 14 nodes
• 24 virtual nodes:
  • each PiLine adds 1 virtual node to the simulation (IEEE14 has 16 PiLines = 16 virtual nodes)
  • each power transformer adds 2 virtual nodes and 3 when considering the resistive losses (IEEE14 has 4 PT = 8 virtual nodes)

[gnakti]

@m-mirz started in #34 to implement additional models to solve this issue.