* is.ncl: doc update

Signed-off-by: Yakkala Yagnesh Raghava <hi@yagnesh.org>
* is.ncl: new function islogicalstring.

Signed-off-by: Yakkala Yagnesh Raghava <hi@yagnesh.org>
* generic.ncl: new function string2int_or_float

Signed-off-by: Yakkala Yagnesh Raghava <hi@yagnesh.org>