Add _Cache to _BaseParameter, use it in GetLatest, remove set_cache method, remove _latest

[astafan8]

Main thing

This PR takes another approach to #1791 - introducing _Cache class which:

• incorporates the _latest dictionary with value, raw_value, and timestamp (also max_val_age), and
• get method that behaves as get_latest and
• set method that behaves as set_cache that was introduced in _BaseParameter cached value: add set_cache method, make _save_val method useless, convert raw_value attribute to property #1757.

This PR subsumes significant part of #1828 but not the improvements to snapshot_base.

Some details of this PR

• GetLatest and get_latest remain as is for backwards compatibility, but under the hood they reuse _Cache and its methods "as-is". In the near future we will be able to deprecate GetLatest altogether
  and move our users to use .cache only.
• Note that _Cache.get as opposed to get_latest.get has an additional get_if_invalid flag that is useful to set to False when you don't want to call get of the parameter if the cached value is invalid (for example, because the parameter has never been captured).
• All docstrings in parameter.py were rewritten to reflect the existence of the _Cache
• Drivers which used set_cache were changed to use cache.set.
• Closes _BaseParameter: add get_cache* API and use it in GetLatest and in snapshot_base #1791 because it subsumes it.
• refactoring test_parameter.py into a package is out of scope for this PR

To be done in another PR:

• rewrite test_parameter.py to test cache.* instead of get_latest.* but also add minor tests for get_latest to ensure that it does the same as cache

[codecov]

Codecov Report

Merging #1832 into master will increase coverage by 0.03%.
The diff coverage is 88.75%.

@@            Coverage Diff             @@
##           master    #1832      +/-   ##
==========================================
+ Coverage   70.24%   70.28%   +0.03%     
==========================================
  Files         148      148              
  Lines       18598    18621      +23     
==========================================
+ Hits        13064    13087      +23     
  Misses       5534     5534

[Dominik-Vogel]

Great! and good job with removing _latest from all the tests!