Issue warning for empty units during semantic analysis

[pramodk]

Empty unit name declaration is valid according to the original implementation in NEURON:

i.e.

UNITS 
{
                   () = (millivolt)
}

is accepted now. Note the empty bracket on the left where there should be a name. Similarly, function declarations like the below are now also accepted:

FUNCTION ssCB(kdf(), kds()) (mM) 
{
}

Above behaviour was implemented in #3 in order to pass the fail existing ModelDB files.

When we will have semantic analysis pass for error checking, we can add a check to see if there is an empty unit definition like above and then error / throw exception. This way, parsing will be successful but the semantic analysis will be a "proper" place to do such checking.

[alkino]

Can you give an example here of such files

[iomaganaris]

On top of that it would be useful to have a more detailed description about what empty units are what they are used for

[nrnhines]

If by empty, you mean no units specified, then that is equivalent to dimensionless, i.e. units syntax of (1).

[alkino]

About empty units in nrn. The truely empty units () is forbidden because:

Redefinition of units () to:	0.001 m2-kg/sec2-coul
 is ignored.
They remain:	1
Error: Units redefinition at line 3 in file a.mod
    () = (millivolt)
                   ^

The onespace unit ( ) is accepted once.

Inside function arguments both are accepted.