Implemented interface for mark mark distribution

[JoseKling]

The interface is implemented. Maybe the names can be improved.

What I implemented:

• An AbstractMarkDistribution type for mark distributions different from the ones in Distributions.jl.
• PointProcessMarkDistribution alias for Union{AbstractMarkDistribution,Distribution}.
• The following methods as part of the common interface.
  • mark_distribution -> The distribution of marks at some time t and after history h.
  • sample_mark -> Sample from mark_distribution. Standard implementation assumes the result of mark_distribution has the method rand.
  • densityof -> for mark m at time t after history h. Standard implementation assumes result of mark_distribution has method densityof.
  • fit -> Fit the distribution to a history h. For processes whose event times are independent of the marks.
  • eltype -> Used to initialize empty histories. Standard implementation assumes result of mark_distribution has method eltype.

What is left

• Add a "tutorial" on how to implement custom processes, explaining the common interface for point processes and mark distributions
• Add new methods to docs. Some docstrings still missing.
• Merge the main branch into this branch