revise categorisation of MVGD

[nesnoj]

The current method of MVGD categorisation uses load density as parameter. There are two essential problems with this:

• Current method disregards the max. length of a MV half-ring (rule of thumb ~1kV/km) which is a central planning principle in MV grids (in a resonant earthed system).
• Load density threshold is an approximate value only.

To estimate needs, current generated grids might be evaluated regarding lengths of half-rings (histogram).

A method considering the issues above:

1. Calculate distances from HV-MV substation to regular LAs
2. Use max. distance (including detour factor) to decide on MVGD type (10kV cable or 20kV line)

[nesnoj]

Done in 937c0d5.
Now setting of voltage level can be done in 2 different ways:

1. Load density (old)
2. Max. distance (new default)

[nesnoj]

It has to be checked how many 20kV and 10kV Grids are resulting.