Hide internal headers

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-Some classes have internal implementation details (e.g., the Checbychev propagator). These should be hidden from user view, not quite sure how. One option would be to put it in the src directory.
+Some classes have internal implementation details (e.g., the Checbychev propagator) or are generally unsuited for direct user interfacing (e.g., FormattingScope). These should be hidden from user view, not quite sure how. One option would be to put it in the src directory.

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-Some classes have internal implementation details (e.g., the Checbychev propagator) or are generally unsuited for direct user interfacing (e.g., FormattingScope). These should be hidden from user view, not quite sure how. One option would be to put it in the src directory.
+There are some header files and classes that are not for consumption by the user, but are exclusively meant for use inside the Wavepacket library. Examples are chebychev_internal.hpp, or the class FormattingScope.
+
+These headers / classes should either not be accessible at all for the user or at least explicitly marked as internal to the library. Possible solutions would be to put the headers into the src/ directory, or extract the classes and put all headers into internal directories, or to add preprocessor macros that prohibit the inclusion of these headers unless specific macros are set.

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 There are some header files and classes that are not for consumption by the user, but are exclusively meant for use inside the Wavepacket library. Examples are chebychev_internal.hpp, or the class FormattingScope.

 These headers / classes should either not be accessible at all for the user or at least explicitly marked as internal to the library. Possible solutions would be to put the headers into the src/ directory, or extract the classes and put all headers into internal directories, or to add preprocessor macros that prohibit the inclusion of these headers unless specific macros are set.
+
+----
+
+Internal headers have been moved to the src/ directory. They are always used exclusively in one package, so referencing them by name only (instead of directory) was sufficient.