Import data of a table without foreign keys show a error.

The error occurs when calling the function `_foreign_key_exists` of the class `abstract_import_manager`

It is an unlikely problem, but occurs if the tables are created without FK (incorrect installation, of course!)
Also will occur if in the future on a new code to import an other table without FK.
manage_import.php fails if aixada runs on a windows server.
Import: Allow deactivate all products of the provider.

To use before importing new products.

NOTE: New method abstract_import_manager->get_match_col_values includes code that is used in some subclasses.
New feature: Templates correspondences to directly import data.
Import using templates: allow update only some column.
Import prevent errors in extreme situations.

  Prevented if the db_table is empty, otherwise the first import may fail.
  Prevent a row without value on a column.
Merge pull request #97 from jorix/i-templates

New feature: Templates correspondences to directly import data.
Merge pull request #94 from jorix/i-extreme

Import prevent errors in extreme situations.
Merge pull request #96 from jorix/i-table_no_FK

Import data of a table without foreign keys show a error.
Import prevent errors in parse_data of data_table.php

  Prevent a row without value on a column.
Merge pull request #98 from jorix/i-extreme2

Import prevent errors in parse_data of data_table.php
Merge pull request #95 from jorix/i-windows

manage_import.php fails if aixada runs on a windows server.