Add an isNumeric() test to record classes

There currently is no straightformat way for generated record classes to check whether a byte pattern contains valid numeric data (in the sense of the COBOL "IS NUMERIC" test). However, this is often needed:

Since COBOL does not really have the notion of an SQL NULL or a Java null, they need other ways to indicate absence of a value. Either a separate flag, or an out-of-band value such as all blanks, or all low-values. For numeric data (if in zoned or external format), that could be a byte pattern that is not in zoned or external format -- that is, a byte pattern for which the COBOL "IS NUMERIC" test would answer false.

So, it would be very useful to have an isNumeric() method on the XxxAccessor interfaces that have the same semantics as IS NUMERIC in COBOL.

This is not really a requirement to the Record Generator but more to the JZOS classes and the com.ibm.dataaccess classes.

What I would propose is the following:

Add a method with signature "boolean isNumeric()" to all "XxxAccessor" interfaces in package com.ibm.jzos.fields. That would be six interfaces (BigDecimalAccessor, BigIntegerAccessor, DoubleAccessor, FloatAccessor, IntAccessor, LongAccessor). Or if possible, add an intermediate "NumericAccessor" interface to the hierarchy which would have the isNumeric() method, and which the six numeric accessor interfaces would extend.

The XxxField implementation classes would then have to implement that isNumeric() method. In the case of "BinaryXxxField", it would trivially always return true since binary fields are alyways numeric by definition.

In the case of, e.g., ExternalDecimalAsBigDecimalField, the implementation would have to check whether the bytes at the buffer position really do represent a number in external decimal format with the respective sign representation. These implementation classes delegate most of the heavy lifting to class com.ibm.dataaccess.DecimalData, which in turn is part of the IBM JDK (I think).
For packed decimals, there is a private utility method, checkPackedDecimal_(), in class com.ibm.dataaccess.PackedDecimal, and that seems to be close to what would be needed. It would basically just need to be made public, and be used from the proposed isNumeric() methods.