Fixes required for O4 SFT/Fscan production

Description

1. MakeSFTs: fix bug in reading non-REAL8 frame channels
   • Must use XLALFrStreamInputREAL8TimeSeries() to support automatic conversion of non-REAL8 frame channels
   • Add test to ensure MakeSFTs can read {Adc|Proc|Sim} REAL{4|8} frame channels
   • Add helper makeMultiChFrame to make non-REAL8 frames
2. spec_avg, spec_avg_long, spec_coherence: check for fopen() != NULL
   • Closes #647 (closed)
3. MakeSFTs: add option to enable/disable validating frame checksums
   • Default is to only validate checksums for --observing-kind=RUN SFTs
   • Closes #664 (closed)

API Changes and Justification

Backwards Compatible Changes

• This change does not modify any class/function/struct/type definitions in a public C header file or any Python class/function definitions
• This change adds new classes/functions/structs/types to a public C header file or Python module

Backwards Incompatible Changes

• This change modifies an existing class/function/struct/type definition in a public C header file or Python module
• This change removes an existing class/function/struct/type from a public C header file or Python module

Review Status

@evan-goetz to review