Passing a vector of solutions now.

preTimestep now accepts a vector of solution together with
a process_id.
In postTimestep only the vector of solutions has been changed.

Fixes multiple copyright messages in the doxygen docu.

ag '\\file' -L --ignore=ThirdParty *Lib |xargs  sed -i 's/\(.*\)\\copyright/\1\\file\n\1\\copyright/'

Most of the current .clang-tidy contained checks:
google-explicit-constructor
google-readability-braces-around-statements
modernize-loop-convert
modernize-use-default-member-init
modernize-use-emplace
modernize-use-using
readability-braces-around-statements
readability-else-after-return
readability-named-parameter
*readability-namespace-comments
*readability-inconsistent-*
readability-redundant-member-init
readability-simplify-boolean-expr
readability-string-compare
*explicit-conversions*

Clang format run on near-the-header lines.

Happy New Year

coupled terms.

and gravitational_term

one variable name, and made some other minor improvements
according to the comments by Dima and Tom.

and made some other minor changes.