Remove `d_prototype` & create type tests for `p_prototype`

[SKopecz]

1. There is no need for a d_prototype, see the discussion in #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?

[codecov-commenter]

:warning: Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

:exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

```diff @@ 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 ```

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

[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

[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