reorder snag fix

Previous reorder init mask additions left the number of inputs mask parameter at 0 and
the init script drawing an empty block for this case. Certain scripts do
not change the number of inputs mask parameter leading to an empty block and
script errors. Default block now with old value of '1' for number of inputs which
should be backwards compatible again with these scripts.
bus_constant added

Allows the construction of a bank of constants whose outputs are
combined into a bus ready for use with other bus blocks.
allow empty divisions in munge block

Some division sizes can now be 0, these will not be included in the output bus.
This is useful if some division sizes might be set to 0 for certain parameter
settings.
Merge branch 'master' of github.com:ska-sa/mlib_devel
Merge branch 'master' of github.com:ska-sa/mlib_devel