docs: added example package

Author: arteevraina

example_packages/preprocess_fypp/README.md

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+# preprocess_fypp
+My cool new project!

example_packages/preprocess_fypp/fpm.toml

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+name = "preprocess_fypp"
+

example_packages/preprocess_fypp/src/file1.fypp

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+#:set LOGLEVEL = 1
+print *, "LOGLEVEL: ${LOGLEVEL}$"

example_packages/preprocess_fypp/src/file2.fypp

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+#:set LOGLEVEL = 2
+print *, "LOGLEVEL: ${LOGLEVEL}$"

example_packages/preprocess_fypp/src/preprocess_fypp.f90

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+module preprocess_fypp
+  implicit none
+  private
+
+  public :: say_hello
+contains
+  subroutine say_hello
+    print *, "Hello, preprocess_fypp!"
+  end subroutine say_hello
+end module preprocess_fypp