SFView for compressed System matrices

[tknopp]

The system function viewer currently does not work since it requires all frequencies to be available. I get this error

julia> SFViewer("/home/knopp/.julia/dev/MPIReco/test/data/mdf/SF1Small.mdf")
Stacktrace:
 [1] error(::┌ Error: Indices applied to systemMatrix are not available in the file
└ @ MPIFiles ~/.julia/packages/MPIFiles/ReVfa/src/FrequencyFilter.jl:151
String) at ./error.jl:33

The solution to this issue is quite clear. We need to first determine what frequency components are available in a calibration file and if one is not we should plot a black image. This also needs to be properly handled in the SNR plots.

[mboberg]

fixed by a2cec2b