Changes review.has_comment to a property

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Changes document.has_comment to a property

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Updates Package docstring.
Implements package.has_optional_field method.
Implements Package checksum, file_name, version, homepage, source_info, summary, description, name, download_location,
verif_code and cr_text validations.

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Creates spdx.checksum.Algorithm to represent checksum algorithms

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Adds file to package and fixes type in write_tv_ex.py

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Adds add_file method to Package class

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Changes tag/value writing example to use package.add_file

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Creates class utils.SPDXNone  to represent SPDX NONE values.
Provides __str__ method for utils.NoAssert and utils.UnKnown
Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Provides __str__ method to creator models.
Changes CreationInfo.has_comment to property
Tag Value writer modifications.
Implements helper functions write_value and write_text_value.
Provides partial implementation of write_package.

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>