Warn the user if pickle is requested but the pickled file is older.

[cuantar]

Whenever usepickle = True is set, the pickled version of a *.tim file will always be used. Consequently, forgetting that the pickled version is in use and then making changes to the *.tim file leads to confusion when the output does not change.

When the pickle file has been modified less recently than the *.tim file for which it can substitute, a helpful advisory message should be produced so that the current state of the program cannot be overlooked easily.

[cuantar]

... it's not the *.tim fle that needs to be checked; it is the config file.

[cuantar]

the warning should not seem dire and scare newbies

[rossjjennings]

This could still be fixed, but we discussed it at a git-flowers meeting and decided that the old PR was too different from the current state of the code, and it would be better to open a new PR based on the current code instead.