optimize PVP for speed

[westram]

Calculation of POS_VAR_BY_PARSIMONY SAI could be optimized similar to parsimony-combine optimization done by [12975] (and preceeding changesets).

Profiling PVP calculation for LTP database showed the following hotspots (in % of overall time; NDEBUG build):

• ~39% spent in "combine"-section​
• ~47% spent in "leaf"-section​

Together that is 88% of runtime.
94% of runtime spent for whole PVP calculation (AP_calc_pos_var_pars).

A factor 2-4x speed-up may be expected for vectorization
⇒ possible overall speed-up: 1.8x-3.2x

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Optimizations to perform:

• reuse passed down memory (i.e. only allocate memory for one recursion, use passed-in-mem for other recursion)
• assert memory is passed down from caller (⇒ remove conditionals)
• optimize code for vectorization