Rework LV grid dimensioning and reinforcement

[nesnoj]

@birgits, @AnyaHe and I identified some issues that need to be changed in the dimensioning and reinforcement of LV grids:

• Use the same cable for entire LV grid
  • Currently, branches are reinforced individually.
  • According to DSO information and the Kerber grids, all cables in a LV grid are of the same type. So for the initial dimensioning (prior to power flow) and reinforcement (after pf) all cable assets should be the smallest possible.
• Introduce a safety factor (in the cfg) and use for the step above
• Use same transformer types in one station during reinforcement
  • Currently parallel trafos of different nom. power are used
  • Change: extend_substation() and extend_substation_voltage()
• Allow for parallel cables in LV reinforcement
  • Currently only the existing branch is replaced or retained if no appropriate cable is available.
  • Change: reinforce_lv_branches_overloading() and reinforce_branches_voltage()

Note: Current reinforcement strategy for LV by @IsGut here: https://github.com/openego/ding0/blob/origin/feature%2Fsynthetic_lv_grids/ding0/grid/lv_grid/build_grid_on_osm_ways.py

[nesnoj]

Related: #366

[nesnoj]

Update: currently simple method by using number of loads for LV cable dimensioning