Update SpicePool hash calculation for mk

At the moment the definition of the SpicePool hash is not consistent between kernels and metakernels:

>>> SpicePool.add('/Users/foo/Desktop/naif0012.tls', purge=True)
>>> hash(SpicePool)
-8796477914650224305

>>> SpicePool.add(('/Users/foo/Desktop/naif0012.tls',), purge=True)  # tuple
>>> hash(SpicePool)
-8796477914650224305  # ✅

>>> SpicePool.add('~/Desktop/naif0012.tls', purge=True)  # with ~ (tilde)
>>> hash(SpicePool)
-8796477914650224305  # ✅

>>> hash(SpicePool.kernels)  # explicit pool list
-8796477914650224305  # ✅

>>> hash(('/Users/seignovert-b/Desktop/naif0012.tls', ))  # explicit paths
-8796477914650224305  # ✅

>>> SpicePool.add('/Users/foo/Desktop/mk.tm', purge=True)  # metakernel file
>>> hash(SpicePool)
-8022798840160630519  # ⚠️

>>> SpicePool.add(Metakernel('/Users/foo/Desktop/mk.tm'), purge=True)  # add MetaKernel object
>>> hash(SpicePool)
9196030471107629286   # ⚠️

>>> hash(MetaKernel('/Users/foo/Desktop/mk.tm'))  # MetaKernel object hash
-8796477914650224305  # ✅

>>> hash(MetaKernel('/Users/foo/Desktop/mk.tm', kernels='~/Desktop/'))  # replace with ~ 
-8796477914650224305  # ✅

>>> hash(TourConfig(kernels='/Users/foo/Desktop/naif0012.tls'))  # TourConfig object with kernels
-8796477914650224305  # ✅

>>> hash(TourConfig(kernels=['~/Desktop/naif0012.tls']))  # TourConfig object with kernels list and ~
-8796477914650224305  # ✅

>>> hash(TourConfig(mk='/Users/foo/Desktop/mk.tm'))  # TourConfig object with metakernel
9196030471107629286   # ⚠️

>>> hash(TourConfig(mk='/Users/foo/Desktop/mk.tm', kernels_dir='~/Desktop'))  # TourConfig object with kernel_dir
9196030471107629286   # ⚠️

mk.tm file:

KPL/MK

\begindata

    PATH_VALUES       = ( '/Users/foo/Desktop' )
    PATH_SYMBOLS      = ( 'KERNELS' )
    KERNELS_TO_LOAD   = ( '$KERNELS/naif0012.tls' )