Complete implementation of new caching system, and extend

[fkuehlein]

New caching was introduced with 93ab748 and merged with #219. See commit message for description and hints on how to proceed.

As a first step towards this:

• The pylint check 'no-member' is currently disabled globally. Working on re-enabling it on 196-review_linting, but would trigger warnings in TestCached as well as RecurrenceNetwork and subclasses. In the latter cases, the old caching is still in place (see above linked commit). Should be reviewed and tested for.
  (EDIT: resolved as of MAINT: extend caching and review Surrogates API #236)

[fkuehlein]

A status update of the task list that came with 93ab748:

Meanwhile completed (as noted above):

• updated classes RecurrenceNetwork and Surrogates (MAINT: extend caching and review Surrogates API #236)

Currently open tasks:

1. Complete implementation

• ClimateNetwork and child classes (i.e. almost the entire climate module) have become children of Cached via the Network class as of 93ab748. ClimateNetwork also has its own __cache_state__() method added. But they do not conform to its protocol, cf. 93ab748 commit message:

  The recursive interaction between __init__(), _similarity_measure and _regenerate_network() across the class hierarchy is very stateful and does not fit into the regular OO dependency pattern assumed by Cached. A redesign of this logic would be advisable, but is left for future work.

2. Extend

Further, but less pressing tasks related to this issue (address as needed):

• add class-specific tests of caching
• extend caching as needed to currently un-cached methods of Cached subclasses.
  (Only remaining classes that are no children of Cached are currently core.Data, core.NetCDFDictionary and funcnet.CouplingAnalysis)

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Note that the Cached class itself has received a little update as of #253.