Add support for fast memory monitoring using smaps_rollup

[amete]

This PR adds the so-called fast memory monitoring option, which uses the smaps_rollup file instead of the standard smaps file, as discussed in #219.

There are some limitation though. Most notably the Size metric is missing in the pre-summed file, where we get the virtual memory measurements, and the fact that smaps_rollup is not supported by some older kernels, e.g., cc7. Therefore, we probably need to do a little bit of testing before using this new mode for anything serious.

Closes #219

[amete]

Wow, I didn't realize this slipped through the cracks for so long @graeme-a-stewart 😄 Thanks a lot for the review. I tweaked the implementation in that if a particular metric is not available, we print a warning message and drop it from the list of monitored metrics. I also fixed the typo in the options. How does this look from a functional point of view? If it looks OK, I can put together a few words in the README for the new functionality and we can probably push this through. Please let me know, many thanks.

[amete]

Thanks a lot @graeme-a-stewart. I just pushed a new version. Can you let me know how this looks, please?