Fix the background output format issue

[Kvieta1990]

Description

When saving the background via Background => Save ... as a .pwdrbck file, the original code would output something like,

#GSAS-II background parameter file; do not add/delete items!
['chebyschev-1', True, 6, np.float(0.054985058337585824), ...]
nDebye:0
debyeTerms:[]
nPeaks:0
peaksList:[]
background PWDR:['', 1.0, False]
autoPrms:{}

This was only the case for Windows. This PR is to fix this issue by converting those numpy float64 numbers to float before output.

Test

Here is a Si project file that can be used for testing,
2023A_GSAS_II.ZIP

[briantoby]

looks fine to me