This commit adds derived types, function pointers, and a filter list
within a core type. These are added to enable support for a filter list
within a core.

This commit adds the mpas_filter_list module, which defines functions
for creating, manipulating, using, and destroying a
mpas_filter_list_type list.

This commit adds a new tool within the context of registry parsing to
extract filter information from an existing registry file. It can be
used to generate filter_function.inc, which can be used to define the
filter's for a core.

It parses filter and filter_list blocks from a registry file, with an
example as follows:

    <filters>
        <filter name="test1" module_name="mpas_test_filter1"/>
        <filter name="test2" module_name="mpas_test_filter2"/>
        <filter name="test3" module_name="mpas_test_filter1"/>
    </filters>
    <filters>
        <filter name="test4" module_name="mpas_test_filter2"/>
        <filter name="test5" module_name="mpas_test_filter1"/>
        <filter name="test6" module_name="mpas_test_filter2"/>
    </filters>

    <filter_list name="group1">
        <filter name="test4"/>
        <filter name="test3"/>
        <filter name="test1"/>
    </filter_list>

    <filter_list name="group2">
        <filter name="test5"/>
        <filter name="test2"/>
        <filter name="test6"/>
    </filter_list>

This commit adds filters to the build system in a shared src/filters
directory. Additionally, this commit adds a driver which may be used to
act on lists of filters. It is provided to allow cores to utilize shared
functionality of a driver, without writing their own custom driver, but
is not required to use filters.

This commit also adds an example filter as
mpas_filter_test_compute_interval. This filter can be used to test that
the compute interval portion of the driver works correctly.

This commit adds filter setup / creation to the MPAS driver, to allow
cores that include filters to have them setup automatically.

This commit adds support to the test core for filters. Additionally, it
adds two filters to the test core that test the compute interval of
filters.