build fix

Version 0.8.1 - release

 Sorted & streamed link transformation - multi and two data set variants

Sorted (& streamed) link transformation - special version for 2 data sets. Relates to ada-discovery/ada-issues#149

Implementation of sorted (& streamed) link multi data set transformation. Relates to ada-discovery/ada-issues#149

Merging from master

added issue tracker page

New widget spec - categorical checkbox introduced.

New widget spec - categorical checkbox introduced (with json serialization). Filter condition resolution fixed. Relates to ada-discovery/ada-issues#134.

Merge data set transformations

Fixing a problem arising when hierarchical categories are used and those that are not referenced by fields are omitted when creating a new data set as a result of a transformation.

New transformations: merge multi data sets with custom mappings and merge multi data sets with all fields matched by name. Closes 17.

Data set transformations: replacing/adding views and filters only if no previous views and filters exist.

Migrating akka utils. Version bump

 Swap-fields data set transformation + independece test (streaming + chi-square fix)

added jacoco code coverage report

Inference performance improved + Filter data-set transformation

Improved field type inference (streaming + transformation abstraction + new tests)