Simplify the (re)use of cached data

With this patch load_cache is extended so it allows for the data fetched to be stored in a file. This file can then be reread later.

Patch consists of an updated Perl module, an updated test script and an added requirement (not sure that last one is needed).

Rationale behind this: I find IEEE quite slow sometimes (even after using the https scheme which is in another patch). So I use the cache quite often but for this I need to fetch the file myself. This is a neater solution.