Adding support of Center-X/Y/Z for VolReader/Writer

# Overview
Actually, `VolReader` & `VolWriter` don't care about Center-X params set in vol files.
It can be problematic if you write a .vol from a centered object and read it after: a shift appears

> Before Writing: [HyperRectDomain] = [[PointVector] {-20, -20, -20}]x[[PointVector] {20, 20, 20}]
> After Reading: [HyperRectDomain] = [[PointVector] {0, 0, 0}]x[[PointVector] {40, 40, 40}]

In this PR, Center-X/Y/Z params are parsed if they are into the vol file and took into consideration.
> Before Writing: [HyperRectDomain] = [[PointVector] {-20, -20, -20}]x[[PointVector] {20, 20, 20}]
> After Reading: [HyperRectDomain] = [[PointVector] {-20, -20, -20}]x[[PointVector] {20, 20, 20}]

_Same for `LongvolReader` & `LongvolWriter`_

# Minor changes
`VolReader.getHeaderValueAsInt()` returns always a value greater than zero (aka failure). This bug is fixed (also in `LongvolReader.getHeaderValueAsInt()`).