Unhelpful error message when a required parameter is not configured

[fmeheust]

Changed parameter parser implementation so that all parameters are added to the parameter set even those for which no value has been set. This allows to get the parameter name to give helpful messages when a parameter is missing.

[Michael-A-McMahon]

I like the removal of ParameterParser.defaultValueSetter, and the cascading removal of code around that field. Less code is always good!
Please see my comments before merging these changes.

[Michael-A-McMahon]

It seems like we lose the defaultValue here; Maybe we want the valueSetter function as:
(value, builder) -> builder.add(name, parameter, value == null ? defaultValue : value)

[fmeheust]

Thank you for catching it!

[Michael-A-McMahon]

In the JavaDoc above, let's add update it to specify that a valueSetter function must be able to handle a null input. I've added in the 2nd paragraph below, and left everything else unchanged.

/**
     * <p>
     * Adds a parameter that a parser identifies by a given
     * {@code name}, and invokes a {@code valueSetter} function for assigning
     * the value. The inputs to the {@code valueSetter} function are the value
     * as it appears in text form and a {@link ParameterSetBuilder} that builds
     * the parsed {@link ParameterSet}. If the name passed to this method is "x", and
     * the parsed input includes an assignment such as "x=0", then "0" is input
     * to the {@code valueSetter}.
     * </p><p>
     * The {@code valueSetter} must be able to handle a null input. A null input will occur
     * when no value is set for the parameter. The {@code valueSetter} may simply pass
     * the null value onto the {@code ParameterSetBuilder}, or it may pass in a default
     * value.
     * </p><p>
     * This method is designed for cases where a single parameter in text format
     * may map to multiple {@link Parameter} objects. The {@code valueSetter}
     * function can perform multiple calls to set each parameter, as in this
     * example:
     * </p>
     * <pre>{@code
     * builder.addParameter("coordinate", (value, parameterSetBuilder) -> {
     *   // Split "x,y,z" formatted value
     *   String[] xyz = value.split(",");
     *   parameterSetBuilder.add("coordinate", X, xyz[0]);
     *   parameterSetBuilder.add("coordinate", Y, xyz[1]);
     *   parameterSetBuilder.add("coordinate", Z, xyz[2]);
     * });
     * }</pre>
     *
     * @param name Name of the parsed parameter. Not null.
     * @param valueSetter Parses and sets the value of parameter(s) from text
     *        input. Not null.
     * @return This builder.
     */

[Michael-A-McMahon]

In the interface class where this method is defined, ParameterSetBuilder.java, we can update the JavaDoc:

  /**
   * Adds a parameter with a given name and value. If the value is null, the ParameterSet will
   * behave as if no value has been set for the parameter. Calling this method with a null value
   * allows the ParameterSet to create error messages that identify the {@code name} of 
   * missing parameters.
   *
   * @param <T> The type of value that is assigned to the parameter
   * @param name The parameter name. Not null.
   * @param parameter The parameter that is assigned with the {@code name} and
   *                  {@code value} in this set. Not null.
   * @param value The value, or null if the parameter is not configured with a value.
   * @return A reference to this object
   */