Changes in 1.5.3
+ fix variable_meta assignment

Changes in 1.5.2
+ use ontology slots instead of STATO

Changes in 1.1.2
+ improved 'show' output for objects
+ allow ANY for entities
Changes in 1.1.1
+ added citations slot to struct classes
+ added corresponding citations method
+ added method to get/set seq_in slot
+ as.SummarizedExepriment now works correctly
+ using seq_in now works for sequences with more than 2 steps

Changes in 1.0.0
+ Biconductor 3.11 release

Changes in 0.99.10
+ update vignettes
+ update documentation
+ add 'signature' input to struct_obj_method

Changes in 0.99.9
+ subsetting of iterators with a single model now converts to a model sequence 
  with 1 step

Changes in 0.99.8
+ documentation changes for Bioconductor checks

Changes in 0.99.7
+ fixed broken is_valid for .outputs
+ minor documentation updates

Changes in 0.99.6
+ .params and.outputs inherited correctly
+ fix some documentation issues

Changes in 0.99.5
+ added as.DatasetExperiment for converting SummarizedExperiment objects

Changes in 0.99.4 (2020-02-04)
+ Set initial value for entity on creation if not provided
+ Rename enum 'list' to 'allowed'

Changes in 0.99.3 (2020-02-03)
+ Improved use of class constructors

Changes in 0.99.2
+ incremental changes in response to bioconductor feedback

Changes in 0.99.1
+ incremental changes in response to bioconductor feedback

Changes in 0.99.0
+ Bioconductor submission

Changes in 0.4.1
+ Final working release before bioconductor submission

Changes in version 0.2.0 (2019-07-25)
+ Introduction of seq_out slot for more flexible workflows (currently method objects only)
