replace float/double conversion with fast_float, update unit tests, update meson

meson.build

@@ -4,5 +4,12 @@ project('ssp', 'cpp',
     'cpp_std=c++17', 
     'buildtype=debugoptimized'])
 
-includes = include_directories('include')
+fast_float_sub = subproject('fast_float')
+fast_float_dep = fast_float_sub.get_variable('fast_float_dep')
+
+ssp_dep = declare_dependency(
+  include_directories: include_directories('include'),
+  dependencies: fast_float_dep
+  )
+
 subdir('test')