dsl: Move SubDimension thickness evaluation to the thicknesses themselves

[EdCaunt]

Fixes issues with standalone use of SubDimension.symbolic_min or SubDimension.symbolic_max in an operator. Previously variants on

x = Dimension('x')
ix = SubDimension.left('ix', x, 2)
f = Function(name="f", dimensions=(ix,), shape=(5,), dtype=np.int32)

eqns = Eq(f[ix.symbolic_max], 1)

op = Operator(eqns)
op(x_m=0)

would suffer from two issues:

1. The SubDimension thickness (ix.symbolic_max) would not be concretised from x_ltkn to x_ltkn0
2. The SubDimension thickness would never be given a value by _arg_values unless the user explicitly passed it to the operator (and even then, it would not be adjusted to reflect any MPI decomposition)

These are rectified by this PR.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.03704% with 4 lines in your changes missing coverage. Please review.

Project coverage is 87.26%. Comparing base (2f4f80f) to head (6e7c017). Report is 21 commits behind head on master.

Files with missing lines	Patch %	Lines
devito/ir/equations/algorithms.py	81.81%	1 Missing and 1 partial :warning:
devito/types/dimension.py	97.82%	1 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2470      +/-   ##
==========================================
+ Coverage   87.22%   87.26%   +0.04%     
==========================================
  Files         238      238              
  Lines       45258    45278      +20     
  Branches     4019     4022       +3     
==========================================
+ Hits        39475    39512      +37     
+ Misses       5103     5085      -18     
- Partials      680      681       +1     

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[FabioLuporini]

Merged, thanks