avoids creating new Unit in often called MVEpoch::getTime()

Avoid creating a temporary MVPosition object in the 3x3 matrix-vector
product by using temporary stack variables.

MVPosition are often created as temporaries where the allocation
overhead of the size 3 vector is large. To reduce this add a very simple
cache for these vectors that holds a couple already allocated vectors.

Euler objects are often created as temporaries where the allocation
overhead of the size 3 vectors is large. To reduce this add a very simple
cache for these vectors that holds a couple already allocated vectors.

the compiler cannot remove then as it must assume memory aliasing.

Returns derivative of MeasTable::posArg and aber*Arg Polynomial,
which is also computed only once and the cached. Avoids unnecessary
Polynomial creations in some places which involved several small Array
allocations.