Some changes to plugins to allow storage of data. Pressure is a plugin now

Samo Penic

2019-02-28 36dba86649e71d2a56657320ddf98a1af2055023

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Original written by Dan Kroll in FORTRAN. Code was then largely adapted and commented by Miha Fosnaric-Fosni. Rewritten into C by Samo Penic-Peni, with help of Miha's extensive explanations.