GUI setting and command line option to ignore checksum rule

[DavidUnderdown]

When dealing with a CSV file that refers to a large quantity of data that requires checksumming that can add a very significant overhead to running CSV validation. It would be useful to be able to turn off checksumming in the validator to allow for quicker checking of other rules.

Of course one can simply remove the checksum rule from the schema, but then it's easy to save the wrong version of the schema. Not setting a path substitution can have a similar effect as the validator simply can't find the files to checksum, but then other reported errors become difficult to find among all the checksum errors.

Possibly this should also extend to the FileExists check too for similar reasons.