Enhance rawhdr to focus on critical header fields for rawspec

[Richard Elkins]

The first .raw file header set of fields should be checked for presence, format, and minimum value:

• OBSNCHAN
• NPOL
• NBITS
• BLOCSIZE
• TBIN
• PKTIDX
• BEAM_ID, optional
• NBEAM, optional
• NANTS, optional
• OBSFREQ
• OBSBW

Value restrictions (rawspec):

• NBITS = 4, 8, or 16
• NPOL = 1, 2, or 4

if the verbose option is selected, all of the header fields will be displayed.