Schema: add tleSystem

This is based on an email from Russell Kackley giving the new
TCS DTD entries for the TLE system:

<!-- A tleSystem (two-line element system) is represented by its orbital
elements. It is primarily used for earth-orbiting objects. Many of the
orbital elements in the tleSystem overlap with those used in the
conicSystem XML element. -->
<!ELEMENT tleSystem (
        epochYr,
        epochDay,
        inclination,
        raanode,
        perigee,
        e,
        LorM,
        n,
        bstar,
        diffRates?)>

<!-- The epoch year of the orbital elements (YYYYY) -->
<!ELEMENT epochYr (#PCDATA)>

<!-- The epoch day (Day of the year and fractional portion of the day) -->
<!ELEMENT epochDay (#PCDATA)>

<!-- The Right Ascension of the ascending node. Units: degrees -->
<!ELEMENT raanode (#PCDATA)>

<!-- The argument of perigee. Units: degrees -->
<!ELEMENT perigee (#PCDATA)>

<!-- The BSTAR drag term. Units: degrees -->
<!ELEMENT bstar (#PCDATA)>