change "check Package" to use needsPackage

This closes issue #127:

    Let's change loadPackage to needsPackage on the command line for
    running an example, and and modify needsPackage so it an error if
    the filename (possibly) requested differs from the one used previous
    time it was loaded.

    This will allow the situation where the package A whose examples are
    being run refers, in its documentation, to a package B that uses A,
    in the case where A is pre-loaded, for example, in the user's
    init.m2 file.

    Anton wants this feature.