[FEATURE] Add option to not provide an initial solution when creating variables

[rb004f]

Is your feature request related to a problem? Please describe.
I have talked with some users who have indicated that it is not always desirable to provide an initial solution for all variables in a problem definition, Currently, many of the variable functions look for a foo_start in the data dictionary, and if not present, a hard coded value is used as an initial value.

Describe the solution you'd like
The request is to consider adding an option to the variable functions to turn off providing an initial solution. This is pervasive across the InfrastructureModels packages, so it may also be considered elsewhere.

Describe alternatives you've considered
N/A

Additional context
N/A

[pseudocubic]

Often it is possible (or in some cases enabled by default) for the solver to ignore the start value of the variables. Is there an example that you can give where this is not sufficient for your needs?

[rb004f]

The specific use case that cropped came from using Knitro, where the user wanted to give an initial solution for a subset of the variables (specifically the binary variables) but not provide an initial solution for the other variables. Since no start values are provided, 0 is set as an initial solution. Kintro checks the initial solution for feasibility, which it is not, and then throws away the entire initial solution. The user tried this outside of PMD (giving a partial solution) and got the desired behavior.