Allow physical units to appear in expressions without a numeric literal?

[clinssen]

Currently, physical units can appear in expressions without a numerical literal in front, for example:

parameters:
    d ms = 1 ms

onReceive(spikes):
    emit_spike(1, d)

will emit a spike with a delay of 1 ms. However, if the user forgets to define the variable or parameter $d$, then NESTML assumes this is the physical unit "days", which has the units of time and thus does not raise any errors or warnings.

@heplesser proposes to allow physical units to appear only directly following a numeric literal, for instance, in case "days" is really what is intended, then:

onReceive(spikes):
    emit_spike(1, 1 d)

would be the correct way to write that.