Additional nomenclature edits

[nfarabullini]

• In legend: Occupational Fund has to be changed to 2nd Pillar, Private Fund -> 3rd Pillar
• In equations: Pillar3 has to become P3

reference issue #103

[riccardoporreca]

@nfarabullini, I would suggest to close this in view of https://github.com/miraisolutions/SmaRP/issues/103#issuecomment-496563260

[riccardoporreca]

Also closing the corresponding placeholder branch feature/128-nomenclature-adj