Automatically set map FITS BLANK keyword

[ossama-othman]

If the source photo FITS file contains a BLANK card/value use that same value for the generated map if the map data type is also a comparable integer. Special handling may be needed for the mosaic case since multiple potentially different BLANK values could exist.

If the BLANK keyword doesn't exist, a value chosen using a procedure that prioritizes speed like the following:

1. Choose a value outside of the [DATAMIN, DATAMAX] range in the FITS file, ideally less than DATAMIN.
2. If neither of those values exist, scan the image array for minimum and maximum array values, and choose a BLANK outside of that range.
3. If the array scan doesn't provide a suitable result, e.g. if the scanned minimum and maximum array values are the same as the image data type minimum and maximum, fall back on a scan with the sky masked out of the image. This step can be time intensive.

[ossama-othman]

MaRC currently reads the source photo data into a C++ std::vector<double> container through the CFITSIO fits_read_pix() library call. Since MaRC supplies a "nulval" argument, CFITSIO will convert BLANK values in integer typed photo data to an IEEE NaN (Not-a-Number) value when writing the read integer values into a floating point array. MaRC should set the corresponding NaN values in the map to the BLANK value found in the source photo FITS file when possible.