Ensure original tags and line numbers are preserved with ramsey_model

In particular, introduce a method for clearing all equations, so that tags and
line numbers are also cleared.

Ref. dynare#1685
6 jobs for master in 8 minutes and 47 seconds (queued for 11 minutes and 57 seconds)
Status Job ID Name Coverage
  Build
passed #12647
build_linux_32

00:00:56

passed #12648
build_linux_64

00:01:40

passed #12651
macOS
build_macOS

00:02:15

passed #12649
build_windows_32

00:01:52

passed #12650
build_windows_64

00:01:59

 
  Prepare
passed #12652
aggregate_builds

00:00:03