Add support for EXPECT x TO BE NUMERIC

[neopragma]

One problem that occurs with older applications is overwriting of packed decimal data items with alphanumeric data, usually when a group item is initialized with spaces, but sometimes for other reasons as well. It's possible developers will want to verify that numeric data items contain numeric data after the program under test performs some operation. So we'd like to be able to write:

Expect [field-name] to be numeric