Add Filter.lambda_eff property
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

dfm/python-fsps,ajmendez/python-fsps,nell-byler/python-fsps,jrleja/python-fsps,bd-j/python-fsps