New random_unit_interval implementation

Re-wrote the random_unit_interval function to generate doubles with 48 bits of random mantissa (3.5e-15), as opposed to the current 32-bit version (2.3e-10).

This implementation uses rand_r (to blend the initial state bytes), and erand48 to generate the result over the initial state bytes. The implementation of both functions is inlined to avoid relatively expensive function calls.

This change passes both the testRandom and testRandomSpacing tests.

doc/RTD/source/index.rst

@@ -21,6 +21,7 @@ difference is the parameter file that will need to be adapted for SWIFT.
    Snapshots/index
    HydroSchemes/index
    SubgridModels/index
+   random
    FriendsOfFriends/index
    EquationOfState/index
    ExternalPotentials/index