Add soversion to shared library

[mgates3]

TestSweeper (and related libraries, BLAS++, LAPACK++, SLATE) have used a date version scheme: yyyy-mm-dd. (At one point, dd was simply a counter starting from 0, but now it is the month day.) This has certain advantages, for instance making it clear when someone is using an out-dated version. However, it doesn't work well for versioning the ABI (application binary interface), which works better with semantic versioning, interpreted as applied to the ABI. The semantic versioning spec is for the API (application programming interface), which can change differently than the ABI. For instance:

• Reordering members of a struct changes the ABI but not the API. Client code needs recompiling but does not need modifications.
• Removing a function changes both the API and the ABI. Client code needs to be updated.
• Changing a templated function's prototype changes the API, but not necessarily the ABI if it is not instantiated in the library. Similarly, changing a macro changes the API but not the ABI.

The GNU libtool manual explicitly says, "Never try to set the interface numbers so that they correspond to the release number of your package."

Therefore, we are versioning the ABI separately from the API, using a form of semantic versioning. We will use the abi-compliance-checker to compare releases of the library. Rules:

• If a public struct is modified or removed, or arguments of a function change (e.g., int to int64), or a function is removed, bump the major version and reset the minor and micro version.
• If a new public structure or function is added, bump the minor version and reset the micro version.
• If changes are only bug fixes or internal structures and functions, bump the micro version.