#8 denull()

[prugniel]

For some reasons the function denull(), and maybe some
other from leda_sql, can desappear from the db
(bug somewhere, dropped by somebody...)
In such a case applications, like interc, may and will
fail with messages which are difficult to understand.

Two things must be done:

1) add a test for the function denull() at the
installation, see in some makefile,
(I think there is a test for a pleinpot_sql function
when leda_sql is installed, which is nonsense, the
functions of leda_sql must be tested).

2) test the sql functions we are using at some point
in interc (probably in the crossid section) and issue a
proper message (telling that the function has not been
properly installed and that should be a bug in the
software installation procedure).