neBEM: fix import of primitives/elements and solution from file
Fix a few issues that caused a crash when importing a previously computed solution from file:
- Non-allocated arrays
- Mismatch between file format (number of variables per line) used for writing and reading elements.
- Charge-zero and floating-conductor constraints not read when parsing the file.
Read in the inverted influence matrix on-demand when requested to compute the weighting field and the matrix is not present in memory.