Allow combinations of variable types

[mdpiper]

Currently, only one type of variable is allowed in an experiment. For example, this:

variables
  uniform_uncertain = 2
    lower_bounds = -1.0 -1.0
    upper_bounds = 1.0 1.0
    descriptors = 'x1' 'x2'

is a variable block that can be produced by this package. However, Dakota supports compound blocks; for example, from the delft3d-polynomial-chaos-1 experiment in mdpiper/dakota-experiments:

variables
  normal_uncertain = 2
    descriptors   'Sand-SedDia'   'Silt-SedDia'
    means               1.0e-4          3.0e-5
    std_deviations     2.28e-5         2.73e-5
    lower_bounds       6.25e-5          8.0e-6
    upper_bounds        2.0e-4         6.25e-5
  uniform_uncertain = 1
    descriptors   'Mud-TcrEro'
    lower_bounds       1.0e-1
    upper_bounds       1.0e+0

We need to update the variables subpackage to allow compound variables blocks to be created.

[mdpiper]

Note there's also an ordering issue, as discussed here.