Dynamic pin mappings support

[nelsobe]

Added support for dynamic pin mappings. Consists of:

1. docs/DynamicPinMapping.txt - description of dynamic pin mappings, how they work, how to create new ones.

2. PinMapping.java - class with code to read and write a dynamic pin mappings cache, search for pin mappings, and create new pin mappings by running Vivado through the VivadoConsole.

3. CreateDynamicPinMappings.java - an example program of how to query for pin mappings and now to create new ones.

4. Miscellaneous support files.

[trharoldsen]

Do I nee to check the type of cell that I need to mapping for is a configurable pin mapping cell or can I call it for any cell and if not a configurable pin mapping cell type, it will return just the default type?

[nelsobe]

@trharoldsen's question above gets to the heart of the issue - we currently do not have a way to conclusively determine which cells have dynamic pin mappings. This PR provides a mechanism to create them and use them, but does not yet provide guidance on how to determine which cells have them.

In #291 we list 9 cells which we know have them. The method for determining this was to determine which cell/bel combinations have unmapped cell pins for the default properties - this would suggest that there are combinations of properties that will cause those unmapped cell pins to become mapped. However, this is a weak test - there may be others as well.

The next step will be to dig deeper into this using this PR's code to help.

Also, a Tcl command has been identified whose name suggests that Tcl may help provide this. However, no documentation has been located yet for this command - the search will continue.