[ccore.nnet.som] O(n^2) memory consumption

**Introduction**
In case of big SOM network sizes memory consumption is significantly big because of O(N*M), where N - amount of rows and M - amount of columns.

**Description**
- Special constant is required to trigger usage of run-time distance calculation or pre-calculated adjacency matrix.
- Pre-calculated adjacent matrix should be used in case if size of the matrix is bigger than defined constant.
- Run-time distance calculation.