Feature/filesystem cleanup (#1587)

* Adapting to the changes in Ghoul
* First step of moving filesystem functions to std
* Remove persistence flag from cachemanager

support/cmake/set_openspace_compile_settings.cmake

@@ -55,12 +55,10 @@ function (set_openspace_compile_settings target)
     # This is disabled until GLM is updated to version 0.9.9 that removes occurrance of this warning
     # "/w44574"   # 'identifier' is defined to be '0': did you mean to use '#if identifier'?
     "/w44608"   # 'symbol1' has already been initialized by another union member in the initializer list, 'symbol2'
-    "/w44619"   # #pragma warning: there is no warning number 'number'
     "/w44628"   # digraphs not supported with -Ze. Character sequence 'digraph' not interpreted as alternate token for 'char'
     "/w44640"   # 'instance': construction of local static object is not thread-safe
     "/w44905"   # wide string literal cast to 'LPSTR'
     "/w44906"   # string literal cast to 'LPWSTR'
-    "/w44946"   # reinterpret_cast used between related classes: 'class1' and 'class2'
     "/w44986"   # 'symbol': exception specification does not match previous declaration
     "/w44988"   # 'symbol': variable declared outside class/function scope
     "/std:c++latest"