Remove d_prototype & create type tests for p_prototype

[SKopecz]

1. There is no need for a d_prototype, see the discussion in Use d_prototype when solving PDSProblems #99. In the algorithm caches we simply use D=similar(u) etc. A PDSFunction still has a field d_protoype which is initialized as d_protoype=similar(u0), but users cannot change this or pass d_protoype as a keyword to a PDSProblem.
2. An additional test ensures that the Ps in the algorithm caches have the same type as a given p_prototype (only for in-place)

[SKopecz]

Does the version change to v0.2.0?

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All modified and coverable lines are covered by tests ✅

Project coverage is 98.09%. Comparing base (258c798) to head (8a7bc3b).

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
- Coverage   98.09%   98.09%   -0.01%     
==========================================
  Files           6        6              
  Lines        1415     1414       -1     
==========================================
- Hits         1388     1387       -1     
  Misses         27       27              

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[ranocha]

Does the version change to v0.2.0?

Yes - this is a breaking change, so we have to increment the leading non-zero digit of the version number. We should also add something like

## Changes when updating to v0.2 from v0.1.x

#### Added

#### Changed

#### Deprecated

#### Removed

- The optional keyword argument `d_prototype` has been removed from `PDSProblem`

to NEWS.md

[ranocha]

Thanks! Could you please increment the version number and describe the changes in NEWS.md?

[SKopecz]

This should now be ready to merge.

[ranocha]

Thanks! I would like to merge #105 first, release a new version of PositiveIntegrators.jl, and merge this PR afterwards