General clean-up of the code for v0.12

[Zigur]

In preparation for v0.12, these code styling improvements are gradually being enforced

• removing asterisk (*) imports in the isatools.create module
• removing asterisk (*) imports elsewhere (isatab.py, isajson.py,...)
• move the constants of the create mode into isatools.create.constants
• move the error messages of the create mode into isatools.create.errors
• remove the following spurious files:
  https://github.com/ISA-tools/isa-api/blob/develop/isatools/create/assay_templates.py,
  https://github.com/ISA-tools/isa-api/blob/master/isatools/create/mtbls-process.py,
  https://github.com/ISA-tools/isa-api/blob/master/isatools/create/study_design_wizard.py,
  https://github.com/ISA-tools/isa-api/blob/master/isatools/create/data.json
• Rename isatools.create.models to isatools.create.model. This will harmonize the naming convention with isatools.model currently singular