Add BigInteger and BigDecimal numerical types

The ImgLib2 library has type-agnostic support for numerical data processing. As such, it would be great to have numerical types based on Java's `BigInteger` and `BigDecimal`, for arbitrary-precision integer- and real-valued numbers, respectively.

Implementing this feature will first require the types to exist within ImgLib2. Then we will need to write additional converter plugins (for the Image > Type menu) as well as ensure that ImageJ2's core numerical commands are well-tested with the new types. Right now, many plugins "cheat" by operating on doubles, which would defeat the purpose of the new types.

Migrated-From: http://trac.imagej.net/ticket/1653

Start by locating the ImgLib2 numerical type definitions, the Image > Type converter plugins, and ImageJ2's core numerical commands mentioned in the issue. Done means BigInteger and BigDecimal types exist, converters support them, and the numerical commands are tested without reducing their values to doubles.

Written by the indexing model from the issue text.