Consolidate in-place implementations for ConservativePDSProblems and PDSProblems (#191)

* single in-place implementation of MPRK22 for ConservativePDSProblem and PDSProblem

* consolidate all in-place implementations in mprk.jl and sspmprk.jl

* removed legacy code

* Apply suggestions from code review

Co-authored-by: Hendrik Ranocha <ranocha@users.noreply.github.com>

* bugfix

---------

Co-authored-by: Hendrik Ranocha <ranocha@users.noreply.github.com>

Author: SKopecz

src/interpolation.jl

@@ -42,11 +42,11 @@ end
 
 #######################################################################################
 # interpolation specializations
-const MPRKCaches = Union{MPEConstantCache, MPECache, MPEConservativeCache,
-                         MPRK22ConstantCache, MPRK22Cache, MPRK22ConservativeCache,
-                         MPRK43ConstantCache, MPRK43Cache, MPRK43ConservativeCache,
-                         SSPMPRK22ConstantCache, SSPMPRK22Cache, SSPMPRK22ConservativeCache,
-                         SSPMPRK43ConstantCache, SSPMPRK43Cache, SSPMPRK43ConservativeCache,
+const MPRKCaches = Union{MPEConstantCache, MPECache,
+                         MPRK22ConstantCache, MPRK22Cache,
+                         MPRK43ConstantCache, MPRK43Cache,
+                         SSPMPRK22ConstantCache, SSPMPRK22Cache,
+                         SSPMPRK43ConstantCache, SSPMPRK43Cache,
                          MPDeCConstantCache, MPDeCCache, MPDeCConservativeCache}
 
 function interp_summary(::Type{cacheType},