Process MGC database tab files from NCBI.

autoSql files created to parse MGC tables:
    mgcClone.as  - clone
    mgcFullength.as - fullength
    mgcLibrary.as - library
    mgcStage1.as - stage1

The NCBI database has columns which allow null values.  This causes
problems when sqlSigned and sqlUnsigned had better error checking.
So that generated parsers were modified by hand to use the gbSql*
parsing functions.

