Add debug switch to capgen (30 hours)

[gold2718]

Add a new option to capgen, --debug, that, when selected, will cause capgen to generate code to test access to "optional" input variables.
The idea is to be able to generate an error before calling a physics scheme.
For the first pass, the only generated check is to test access to a variable which has the active keyword in its metadata. The test would be performed when the active clause is .true. as in the example below (copied from the 29 September meeting minutes):

[ntwa]
  standard_name = index_for_water_friendly_aerosols
  long_name = tracer index for water friendly aerosol
  units = index
  dimensions = ()
  type = integer
...
[qgrs(:,:,index_for_water_friendly_aerosols)]
  standard_name = water_friendly_aerosol_number_concentration
  long_name = number concentration of water-friendly aerosols
  units = kg-1
  dimensions = (horizontal_dimension,vertical_dimension)
  active = (index_for_water_friendly_aerosols > 0)
  type = real
  kind = kind_phys

--> in the cap:

if (ntwa > 0) then
   dummy = sum(qgrs(:,:,ntwa))
end if
! now call subroutine that does something with qgrs(:,:,ntwa) ...

[climbfuji]

For the sake of completeness, the default value for the attribute active (if not specified in the metadata) is .true., i.e. all these variables should be checked prior to entering a scheme.

[climbfuji]

We should test array allocations and dimensions for all arrays that are supposed to be present, i.e. also those that have an implicit/default active = true value. Doesn't cost anything more.

[climbfuji]

Closed via #512