Implement zeroPad string provider

[DavidUnderdown]

In the case of our digitisation project, the file_path includes a zero-padded running number (aka ordinal) for the image files within a piece or item. Using concat within starts and ends expressions, plus some regex allows fairly tight validation of the fact the file_path has been built up as expected eg

file_path: starts(concat("file:///",$department,"",$series,"/",$piece,"/",$item,"/",$piece,"",$item,"")) and regex("^.*[0-9]{1,4}.jp2$") and ends(concat($ordinal,".jp2"))

but there is still a small chance that there might be some digit other than zeroes before the ordinal at the end of filename (before the jp2 extension).

if we had a zero padding expression, something like zeroPad($ordinal,4) would return a string provider padded with leading zeroes sufficient to make the string 4 characters long, eg zeroPad(5,4) would produce 0005 while zeroPad(10,4) would give 0010 and zeroPad(104,4) would give 0104.

A full validation of the file_path might then look like:

file_path: starts(concat("file:///",$department,"",$series,"/",$piece,"/",$item,"/",$piece,"",$item,"_",zeroPad($ordinal,4),".jp2"))