FILTER.msun_ab and FILTER.msun_vega properties

These properties provide lazy-loaded access so the solar absolute
magitudes in $SPS_HOME/data/magsun.dat The datafile is parsed with numpy
and cached as a module-level variable MSUN_TABLE
Add Filter.lambda_eff property

Property accessor to a filter's effective wavelength in Angstroms.

Like the solar magnitudes, the data is lazy loaded into the module
namespace after parsing the file at:
$SPS_HOME/data/filter_lambda_eff.dat