- minor simplification of #if/#endif handling
ENH: improve input robustness with negative-prefixed expansions (#2095)
- especially in blockMeshDict it is useful to negate an value directly.
Eg,
```
xmax 100;
xmin -$xmax;
```
However, this fails since the dictionary expansion is a two-step
process of tokenization followed by expansion. After the expansion
the given input would now be the following:
```
xmax 100;
xmin - 100;
```
and retrieving a scalar value for 'xmin' fails.
Counteract this by being more generous on tokenized input when
attempting to retrieve a label or scalar value.
If a '-' is found where a number is expected, use it to negate the
subsequent value.
The previous solution was to invoke an 'eval':
```
xmax 100;
xmin #eval{-$xmax};
```
which adds additional clutter.
