Configure max number of model instances

[mdpiper]

To allow multiple instances of a model to be instantiated (think @wk1984's implementation of GIPL as a use case), the babelizer Fortran interoperability layer has a parameter

integer, parameter :: N_MODELS = 2048

For models that use statically allocated variables, this presents a problem--2048 copies of a model with memory already allocated are created! This, I believe, is at the heart of the problem in csdms/help-desk#104.

In this PR, an undocumented max_instances parameter is added to the build section of the babelizer input file. It's set to a small default number (currently 8), which allows enough instances to be created to run the bmi-tester. Users can override this default value in the babelizer input file; e.g.:

[build]
undef_macros = []
define_macros = []
libraries = []
library_dirs = []
include_dirs = []
extra_compile_args = []
max_instances = 32

or

[build]
undef_macros = []
define_macros = []
libraries = []
library_dirs = []
include_dirs = []
extra_compile_args = []
max_instances = 1

I'm choosing to keep this new parameter undocumented, pending further study.

[mdpiper]

@lzhu5 This is the change I'm plotting for the babelizer.

[mdpiper]

I should parameterize the tests like @mcflugen did in #60.