'hist' shall use categories, if available

[numeredev]

SUMMARY

Implements necessary changes for #26

Reviewers: @numere-org/maintainers

IMPLEMENTATION

• Implementation: Implemented the usage of categories and logical values as described by the issue. Categories not representable by a common superset will be represented by abstract categories
• Implementation test: Tested single categorical and logical columns as well as a combination of multiple categories (not) representable as common superset.

DOCUMENTATION

• [x] ChangesLog updated
• [x] Code changes commented
• Documentation articles:
  • [ ] corresponding documentation articles updated
  • [ ] new documentation articles created
  • [x] not needed -- will be done with the changes required by #27
• Language files:
  • [ ] corresponding language files updated
  • [x] not needed

---

TESTS BY REVIEWERS

• [x] Added to the automatic SW tests
• [x] Tested manually

[numeredev]

To test this change

new categories(), result() -free
## Create some categories
categories(:,1) = {"Hello", "World", "World", "Hello", "Hello"};
categories(:,2) = {"Hello", "Hello", "World", "Hello", "World"};
categories(:,3) = {"A", "B", "A", "C", "D", "A", "C"};

## Add headlines
categories(#,:) = {"Hi", "Hi2", "Alpha"};

## Convert strings to categories
categories().convert({1:2}, "category");

## Test one category
hist categories(:,1) -set target=result()
assert result(:,1) == {"Hello", "World"};
assert result(:,2) == {3, 2};

## Test also category mixtures with two columns and three columns:
## Assumption is: column 1 and 2 result in the same categories, column 1,2,3 together will use 
## generic categories like "Cat:1", "Cat:2", ...