annualSummary re-written to handle dates more effectively.

A separate dates argument is included to handle non-Landsat data, or data lacking a date dimension or appropriate layer names. Support for a z-dimension including dates is now included.

Also, there was a conflict with the years argument and an internal variable which has now been solved.
Added support for z-dimension

This is implemented in functions where a dates vector is required (either explicitly supplied or derived from the z-dimension or names(x)).

Also, doMC was removed from @import and replaced with foreach, and also replaced in DESCRIPTION to prevent the error in roxygenise().

Package was finally roxygenise()'d after all changes
Merge branch 'feature-getZ' into develop