ddl2cpp: Cleanup part 3

[MeanSquaredError]

This is the third PR that cleans up the ddl2cpp code. It provides the following changes:

• Wraps all the global DDL parser variables into a separate class (DdlParser). This pretty much gets rid of all the global variables that were polluting the global namespace.
• Cleans up the code that handles custom data types. The previous version seemed a bit hacky since it was modifying directly the global lists that contained the variable type names.
• Replaces a for-loop with a list comprehension.
• Clarifies the error message that is displayed when an unknown SQL data type is found in a DDL file. The previous error message stated:

Error: unsupported datatypes.
Possible solutions:
A) Implement this datatype (examples: sqlpp23/data_types)
B) Use the '{dataTypeFileArg}' command line argument to map the type to a known > type (example: README)
C) Extend/upgrade sqlpp23-ddl2cpp (edit types map)
D) Raise an issue on github

There we several problems with this:

• Solution A (implementing the datatype in sqlpp23) or C (adding the data type in sqlpp23-ddl2cpp) would not work alone, because one needs to modify both sqlpp23 and sqlpp23-ddl2cpp for the new type to be recognized. So I've merged A and C into a single solution.
• Solution B (mapping the unknown data type to a known one) would definitely work, so I made it the first proposed one.
• Solution D (Raise an issue on github) will work so I left it as the last proposed one.

Also the message did not make a difference between an SQL data type and an sqlpp23 data type and in fact these are different data types, an SQL data type is just a keyword, recognized by the database, e.g. INT, while the sqlpp23 data type is a C++ structure, e.g. sqlpp23::integral. So I tried to differentiate these two terms to make it clear that both should be implemented.

In the end the proposed error message looks like:

Error: unsupported SQL data type(s).
Possible solutions:
A) Use the '--path-to-datatype-file' command line argument to map the SQL data type to a known sqlpp23 data type (example: README)
B) Implement this data type in sqlpp23 (examples: sqlpp23/data_types) and in sqlpp23-ddl2cpp
C) Raise an issue on github

I tested the PR creating a single header, split headers and a module, each of these with table creation helpers (--generate-table-creation-helper) and it seems to work correctly. Also tested DDL definitions with custom types and --path-to-datatype-file and this worked correctly too.

[rbock]

Thanks for the continued cleanup!