- 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.