Allow relbin's fiducial parameters to not be floats

[cdcapano]

Currently the Relative model tries to cast all of the reference parameters specified in the model section to floats. This is a problem if you want to pass in, say, a string (like a different approximant name than what's in the static params) or another parameter type to be passed to the waveform generartor. This patch fixes the issue by first checking if it's a boolean parameter, then catching any attempt to cast to float that fails.