Implement dynamic increases (and more general, sub-blocks of contracts, potentially with different parameters)

-) Rearrange InsuranceContract to split functions into public and private functions
-) InsuranceContract: Return invisible(self) in as many functions as possible...
-) Implement InsuranceContract$addBlock and InsuranceContract$addDynamics to add new sub-blocks to the contract
-) Properly implement consolidating values from child blocks (parent blocks have some values like unit cash flows erased, as they do not make sense!)
-) Add contract/block id to assign each block a readable, unique ID
-) Extend exportInsuranceContract.xlsx to export the data.frames of the contract and all sub-blocks (not yet finished, in particular overview page and PV with premium calculation)
-) Write out values for  all contract blocks to Excel
-) history entry for dynamic increases
-) translate Excel export
-) Excel export: Make cost table generic (loop through all contract blocks)
-) Fix some UTF-8 issues in Windows
-) Export to Excel: Clean table names; clean endowment example; Rebuild docs

Fixes issue #35, relates #48.