Handle no data in BandMath and BandMathX

Requested by multiple users: #1511 (closed), #1801, otb-users

Better handling of images with no-data in BandMath and BandMathX:

• A flag to ignore no-data values (maybe even on by default?)
• A nodata token available in the expression that is set to the nodata value reported by gdal
• support nan() function in BandMathX #1801