#47 A few fixes and changes in blt::vector

[Sergei Golovan]

Hi!

1) It looks like two vector operations have bugs in BLT 3:
* skew(x) is defined incorrectly as a normalized mean of the absolute value of x^3, there shouldn't be an absolute value (Ex^3/s^3, not E|x|^3/s^3). This bug is present in BLT 2 as well.
* q2(x) is defined as mean(x), but should be median(x). There's no q2() in BLT 2.
2) Looking into tests/vector.tcl, I've found the following failing test [blt::vector create z_values(50)]. Since specifying vector length as (50) or first/last indices as (1:50) was the intended way of creating preallocated vectors in BLT 2, I've tried to reimplement it for BLT 3 as well. Also, I've added options -first and -last (they were listed in the comments in bltVectors.c, so I assume that it's the new way to specify the range of indices).

Please, consider the attached patch with the following changes:
1) src/bltVecMath.c: fixes bugs with skew() and q2().
2) src/bltVectors.c: adds -first and -last options for vector creation. Also, changes the type of -length to BLT_SWITCH_INT_POS because the size have type int (so BLT_SWITCH_LONG_POS overwrites switches.first).
3) src/bltVectors.c: Moves parsing name(12) and name(1:10) to a separate function, which is called both from VectorCreateOp() and OldVectorCreate().
4) src/bltVectors.c: changes size = last - first to size = last - first + 1 on vector creation. Probably, you wanted ranges like 1:3 mean 1 2, but this would require the respective changes in other places where ranges are acceptable (like ::vector value set 1:3 1 which currently sets values for 3 elements, and not for 2. Clearly, this change can be discussed further.
5) src/bltVectors.c: implements defining vector size and offset in [blt::vector create] by both (23:42) and -first 23 -last 42'. 6) src/bltVectors.c: implementsv(:)as a synonym tov(all). It didn't work as it was before for the case of an empty vector. 7) tests/vector.tcl: fixes all the existing tests and adds a few tests for[blt::vector create]`.