Adds test_data to .gitignore

Changes to files in test_data must be made forcefully.
Fixes read-only support

Addresses a couple spots where opening the coverage in read-only ('r') mode wasn't being respected properly.
Adds support for fixed length and constant strings

Updates the ConstantType parameter type so that it can support constant strings.  Also makes some improvements to efficiency and flexibility.

Ensures support for fixed-length strings (via QuantityType)

Updates related tests appropriately