Merge pull request #2 from has2k1/fix-style-and-readability

Remove tabs and use descriptive names
Added types.f90: contains type definitions
Implemented new type definitions main and finite_difference, updated Makefile
Started documentation, added build files to .gitignore
Remove integer type

- At the moment only double precisions need a type, i.e. `_dp`
  This type is meant to be appended to all numeric assignments to
  double precision variables and numeric quotients of divisions.

- Fix an integer division in main