Build kernel from input.cfg

[cglosser]

Because QuEST is so marvelously generalized across propagator types, we should add the option for users to specify which propagator they'd like to use in the input file. Essentially, the enhancement here is to

1. Add a shared(? maybe unique) pointer to the Configuration struct. This pointer will hold the kernel we're eventually going to build.
2. Read the kernel type (as text) from input.cfg
3. Read the kernel parameters from input.cfg (this is trickier as different kernels have different numbers of parameters)
4. Assemble the parameters into the appropriate kernel at the end of parse_configs.

Step 4 will be the spot to add a switch statement that switches on the kernel type, but if we do things right that should be the only place we need to maintain such a field.