File set declaration for XProcScripts

Input and output filesets can be declared in the script file:
```xml
<p:declare-step version="1.0" name="unit-test-script"
       type="px:unit-test-script"
       px:input-filesets="dtbook epub3"
       px:output-filesets="zedai html"
       xmlns:p="http://www.w3.org/ns/xproc"
       xmlns:px="http://www.daisy.org/ns/pipeline/xproc"
       xmlns:xd="http://www.daisy.org/ns/pipeline/doc"/>
```

and they are exposed in via the api in this fashion:
```xml
<script href="http://localhost:8182/ws/scripts/dtbook-to-html" id="dtbook-to-html" input-filesets="dtbook dtbook2.02" output-filesets="html html5">
```