Shim class for loading G3SuperTimestream objects from disk (#192)

This PR is the first step in integrating the G3SuperTimestream class used by the Simons Observatory team into the core library.  This class is a very thin wrapper around the core G3TimestreamMap class.  It is designed only to load G3SuperTimestream objects from G3 files on disk for immediate conversion to G3TimestreamMap objects.  It is also not exposed to the public or python API.

Detector names maintain their ordering from file.  Missing samples are gap-filled with zeros (integer data) or nans (float data).  For integer data, missing samples are also marked with an extra `"_nanmask"` detector in the data, wherein 1=missing sample, 0=good data.  Data are decompressed immediately on read, using openmp for effiicency.

Author: arahlin

core/CMakeLists.txt

@@ -5,6 +5,13 @@ else(APPLE)
 	set(CORE_EXTRA_SRCS "")
 endif(APPLE)
 
+if(NOT DEFINED WITHOUT_SUPERTIMESTREAM)
+	set(WITHOUT_SUPERTIMESTREAM FALSE CACHE BOOL "Build G3SuperTimestream class")
+endif()
+if(NOT WITHOUT_SUPERTIMESTREAM)
+	set(CORE_EXTRA_SRCS ${CORE_EXTRA_SRCS} src/G3SuperTimestream.cxx)
+endif()
+
 add_spt3g_library(core SHARED
 	src/G3Logging.cxx src/G3PrintfLogger.cxx src/G3SyslogLogger.cxx
 	src/crc32.c src/G3Frame.cxx src/G3Data.cxx src/G3TimeStamp.cxx
@@ -40,6 +47,13 @@ if(FLAC_FOUND)
 	target_link_libraries(core PRIVATE FLAC::FLAC)
 endif()
 
+if(NOT WITHOUT_SUPERTIMESTREAM)
+	find_package(OpenMP QUIET)
+	if(OpenMP_FOUND)
+		target_link_libraries(core PRIVATE OpenMP::OpenMP_CXX)
+	endif()
+endif()
+
 # Link against Z library
 if(NOT DEFINED WITH_ZLIB)
 	set(WITH_ZLIB TRUE CACHE BOOL "Enable gzip fie compression")