PyFoam Merge Request #1: Possible overestimation in memory usage for linux (open)

Hello. I have been using pyFoam for carrying out some parametric studies and I faced some inconsistent results using distinct memory usage metrics. After reading this code and reading some about memory benchmarking, I think the algorithm implemented is possibly overestimating memory usage. From linux kernel documentation (https://docs.kernel.org/filesystems/proc.html):
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The “proportional set size” (PSS) of a process is the count of pages it has in memory, where each page is divided by the number of processes sharing it. So if a process has 1000 pages all to itself, and 1000 shared with one other process, its PSS will be 1500. “Pss_Dirty” is the portion of PSS which consists of dirty pages. (“Pss_Clean” is not included, but it can be calculated by subtracting “Pss_Dirty” from “Pss”.)
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In that way, PSS already take into account private memory usage, such that we need to sum only PSS values to get current the memory usage of a parallel run. Psutil library also implies this same behavior to me.

In my tests, I have been reading from /proc/pid/smaps_rollup, which is the same method that psutil library use when running on a kernel with this feature (https://github.com/giampaolo/psutil/blob/master/psutil/_pslinux.py#L1978). So, in my tests, this modification make the results consistent again. Thus, I am creating this merge request for your evaluation.

Thank you for your time and this wonderful library.