Redshift wavelength calculation now done in double precision

[pllim]

Redshift wavelength calculation now done in double precision. Otherwise, some wavelengths are merged unnecessarily, resulting in loss in sampling. Example:

>>> import pysynphot as S
>>> from pysynphot.spparser import parse_spec
>>> obsmode = 'wfc3,uvis1,f555w'
>>> spectrum = ('rn(z(spec($PYSYN_CDBS/calspec/bd_28d4211_stis_001.fits),'
...             '0.1),band(johnson,b),28.0,vegamag)')
>>> sp = parse_spec(spectrum)
>>> bp = S.ObsBandpass(obsmode)
>>> obs = S.Observation(sp, bp)
>>> obs_ref.wave[11044:11049]  # AFTER this fix
array([ 8721.        ,  8722.        ,  8722.39453125,  8722.39467773,
        8723.        ])

Without this fix, 8722.39453125, 8722.39467773 are merged as 8722.39453125 under single-precision calculations.