dsl/compiler: Fix&tidy PETSc mixed solver functionality

[ZoeLeibowitz]

A portion of this PR involves code reorganisation and general cleanup. Key additions:

• PetscBundle inside petsc/types/array.py
• Various new types inside petsc/types/types.py to help simplify PETScSolve
• PetscBundle use inside petsc/iet/routines.py to fix mixed solver functionality
• A mixed solver test/example - 02_biharmonic.py

[codecov[bot]]

Codecov Report

:x: Patch coverage is 31.80760% with 879 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 84.69%. Comparing base (6b6ad4f) to head (ff4b2d7). :warning: Report is 61 commits behind head on petsc.

Files with missing lines	Patch %	Lines
tests/test_petsc.py	12.69%	330 Missing :warning:
devito/petsc/iet/routines.py	9.04%	191 Missing :warning:
devito/petsc/types/types.py	30.31%	177 Missing :warning:
devito/petsc/logging.py	38.27%	50 Missing :warning:
devito/petsc/iet/passes.py	27.90%	30 Missing and 1 partial :warning:
devito/petsc/solve.py	27.77%	26 Missing :warning:
devito/petsc/types/array.py	54.71%	24 Missing :warning:
devito/petsc/iet/logging.py	34.28%	23 Missing :warning:
devito/petsc/iet/utils.py	27.77%	13 Missing :warning:
devito/operator/profiling.py	68.75%	4 Missing and 1 partial :warning:
... and 5 more

Additional details and impacted files

@@            Coverage Diff             @@
##            petsc    #2626      +/-   ##
==========================================
- Coverage   85.39%   84.69%   -0.70%     
==========================================
  Files         261      265       +4     
  Lines       50604    51307     +703     
  Branches     4326     4351      +25     
==========================================
+ Hits        43211    43453     +242     
- Misses       6667     7126     +459     
- Partials      726      728       +2     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.