added features to get index of files in `CdsHeader.write` [skip ci]
added features to get index of files in `CdsHeader.write`
[skip ci]

mhvk/astropy,saimn/astropy,larrybradley/astropy,astropy/astropy,mhvk/astropy,astropy/astropy,lpsinger/astropy,lpsinger/astropy,astropy/astropy,lpsinger/astropy,StuartLittlefair/astropy,StuartLittlefair/astropy,saimn/astropy,astropy/astropy,StuartLittlefair/astropy,larrybradley/astropy,pllim/astropy,saimn/astropy,StuartLittlefair/astropy,pllim/astropy,lpsinger/astropy,StuartLittlefair/astropy,mhvk/astropy,mhvk/astropy,pllim/astropy,lpsinger/astropy,pllim/astropy,saimn/astropy,astropy/astropy,pllim/astropy,larrybradley/astropy,saimn/astropy,larrybradley/astropy,mhvk/astropy,larrybradley/astropy