feat: flexible stock and initial stock

[JakobBD]

Purpose of this PR

This could be broken into two PR's, where f18d93a is the first commit of the second PR:

Flexible stock

Adds a FlexibleDSM class which uses either stock driven or inflow driven compute routine, depending on what you pass it.
Deletes the lapack solver on the way.

TODO (apart from the usual):

• Make it throw an error if both are set.
• Export FlexibleDSM in init (check line from compute_processes branch)

Deleting the lapack solver is a break of backwards compatibility, which seems fine, as I doubt anyone is using it.

Initial stock

• Add initial stocks to DSM
• Add possibility to differentiate inflow by cohort (used product imports)
• Retrieve stock_by_cohort and outflow_by_cohort as flodym arrays

TODO (apart from the usual):

• check TODO general points in stocks.py inline comments (copied here):
  • np allclose in all
  • make work for inflow_by_cohort?
  • proper treatment of prescribed inflow and stock at initial year
    (for inflow: only represents/replaces the newest cohort!)
  • add inflow_by_cohort to FlexibleDSM
  • move adapt_stock / adapt_inflow to respective subclasses
• check for backwars compatibility

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Builds on #124, so that should be merged first

Type of change

(Make sure to delete from the Type-of-change list the items not relevant to your PR)

• Bug fix
• Refactoring
• New feature
• Minor change
• Major change

Checklist:

• I have updated the in-code documentation of all changed classes and functions
• I have added in-code documentation and type hints to all new classes and functions
• I have adapted the howtos
• I have adapted the examples
• If this change should entail a version update, I have bumped the version in the pyproject.toml file accordingly