Reorganise code and documentation for mapping tables.

[inpowell]

This pull request splits R/mapping_table.R into separate files:

• R/BaseMappingTable.R contains definitions for the MappingTable and BaseMappingTable classes.
• R/MultiMappingTable.R contains the definition for the MultiMappingTable class.
• R/RangeMappingTable.R contains the definition for the RangeMappingTable class.

Given that the (so-called) "internals" of the classes might be prone to change, the tests for each of these classes have been whittled down.

Documentation for all MappingTable subclasses has been improved, preventing warnings from roxygen2 when documenting classes. Examples have been added for all classes in the documentation.

[inpowell]

(Hint: git diff --color-moved origin/master or similar with recent versions of git really helps understand what has changed, given the file-renames that have occurred.)

[PeterM74]

I have added a commit.

• The mapping table documentation was duplicating the title into the description, I have fixed this by specifying the @title and @description settings explicitly rather than letting roxygen (incorrectly) guess.
• Removed .Rd for the to_dense() function as it's not exported. From my review, there is no need to output documentation for non-exported functions for CRAN checks so I think this should be ok.
• Some other minor spelling/grammar fixes.

Local check ok
── R CMD check results ────────── modulartabler 0.1.0 ────
Duration: 36.5s

0 errors ✔ | 0 warnings ✔ | 0 notes ✔

[inpowell]

Thanks for the pick-ups!