Make sure doubles aren't promoted to 16 bytes (#652)

# CABLE

## Description

Turns out just using `-fdefault-real-8` also tries to promote any double
declarations to 16 byte precision e.g. `1.0d0`. Specifying
`fdefault-double-8` supposedly fixes this. Sorry to bug you again
@SeanBryan51

Fixes #(560)

## Type of change

- [x] Bug fix

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📚 Documentation preview 📚:
https://cable--652.org.readthedocs.build/en/652/

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Author: Whyborn

CMakeLists.txt

@@ -134,7 +134,7 @@ if(CABLE_LIBRARY)
         if(CMAKE_Fortran_COMPILER_ID MATCHES "Intel")
             add_compile_options(-r8 -i8)
         elseif(CMAKE_Fortran_COMPILER_ID MATCHES "GNU")
-            add_compile_options(-fdefault-real-8 -fdefault-integer-8)
+            add_compile_options(-fdefault-real-8 -fdefault-integer-8 -fdefault-double-8)
         else()
             message(STATUS "Cannot compile library with unknown compiler")
         endif()