ORC-531. Refactor schema evolution including removing the float conversions.

[omalley]

This PR removes about 700 lines from the schema evolution code:

• Removes most of the float conversions, which aren't needed now.
• Removes the duplicate reference to the underlying TreeReader in each subclass.
• Creates a new double->float conversion that casts it to a float instead of using strings.
• The test case float->double had to loosen the delta on the comparison since we don't get the implicit truncation from going through stringification.
• Moves the file names for TestSchemaEvolution to use that as the filename prefix on the written orc files.