Unit test `calc_fee`; Refactor `fetchBlock` (#187)

* Refactor partial fee calculation to make it testable

Correct name

Create mock api skeletion

save changes

refactor fetch blocks

* Add dummy header

* test createCalcFee

* More refactors of fetchBlock

* Rename constants.ts => mock.ts

* Move mockApi to mock file

* Move header to its own file

* Update commetn

* Update comment