More work towards 32 bit distance matrices

[micheles]

And logging a warning when the spatial correlation matrices are degenerate. Unfortunely, the memory occupation is the same, but we are a bit faster. Here an example for JPNconditioned:

# before
| calc_7616, maxmem=5.8 GB     | time_sec | memory_mb | counts |
|------------------------------+----------+-----------+--------|
| total ebr_from_gmfs          | 340.5    | 495.8     | 8      |
| EventBasedRiskCalculator.run | 341.4    | 742.5     | 1      |
| computing risk               | 239.2    | 0.0       | 1_632  |
| total conditioned            | 221.4    | 125.3086  | 1      |
| computing cov_Y_Y            | 181.1    | 2_125     | 4      |
# after
| calc_7615, maxmem=5.8 GB     | time_sec | memory_mb | counts |
|------------------------------+----------+-----------+--------|
| total ebr_from_gmfs          | 334.2    | 501.0     | 8      |
| EventBasedRiskCalculator.run | 329.7    | 662.2     | 1      |
| computing risk               | 234.8    | 0.0       | 1_632  |
| total conditioned            | 213.1    | 69.9609   | 1      |
| computing cov_Y_Y            | 174.2385 | 2_125     | 4      |