ThousandSep - Number as string with separators every 3rd digit
S = ThousandSep(N, FSpec, Sep)
INPUT:
N: Numbers, all classes accepted by SPRINTF, scalar or array.
FSpec: Format specifier for SPRINTF. Optional, default: '%.16g'
Sep: Character as separator. Optional, default: ,
OUTPUT:
S: String if N is a scalar, otherwise a cell string.
EXAMPLE:
ThousandSep(1234567.2345) % '1,234,567.2345'
ThousandSep(1234, char(39)) % '1'234'
ThousandSep([2.3, 1234], ',') % {'2.3', '1,234'}
ThousandSep(-1234, '%9.1f', ',') % ' -1,234.0'

NOTES:
* Alternative java call, which is about 20% slower (Matlab R2011b/64/Win7):
nf = java.text.DecimalFormat;
S = nf.format(1234567.890123)
* Matlab 6: The code must be modified to run under Matlab 6. See the comment
in the subfunction ToString.

Tested: Matlab 7.7, 7.8, 7.13, WinXP/32, Win7/64