Track engine hours per engine

[wlbragg]

Fixes #1424

@dany93 please test.

The only other thing I would like to verify is if we can delete the unused filter in Systems\instruments.xml. Starting at line 189.

<filter>
        <name>Engine Total Hobbs Hours</name>
        <type>gain</type>
        <input>
            <expression>
                <div>
                    <sum>
                        <property>/sim/time/hobbs/engine[0]</property>
                        <property>/sim/time/hobbs/engine[1]</property>
                    </sum>
                    <value>3600.0</value>
                </div>
            </expression>
        </input>
        <output>
            <property>/instrumentation/clock/hobbs-meter-hours</property>
        </output>
    </filter>

If is is indeed not being used anywhere I want to delete it before merging.

@wkitty42 maybe you can help verify it is not being used anywhere?

[wlbragg]

@dany93 This issue is ready to merge after review. There is no reference to the removed filter in any of the code. So I felt it was safe to remove.