Mpfi

Fix 38 Make variable type checking optional
Fix 39 Export fails when a string column is all empty (due to length checking for variable type)

Critical Update

- config file will be fully deprecated in the near future.
- `load_mpf` will replace the original `load` and `load_all` functions
- `export_mpf` will replace the original `export` function
- `load`, `load_all` and `export` should still be functional in this version but will be removed in the next release
- `load_fac` function is not renamed, but it does not read the FAC extension from the config anymore.

Fix 34 Avoid parsing rubbish footer lines in `load_fac`

35 Fix bug when MPF file does not have header lines

32 Handling D (date type) in MPF files with only minimal support

31 Force export variable type string length to integer, i.e. `T1` instead of `T1.0`