Feat/yuanrui/enhanced images

[zhang-yuanrui]

Consolidate the first version of the enhanced image module.

What it can accomplish:

1. Given a vtk polydata and a certain variable in the polydata, it can generate an enhanced image.
2. Given a DPF model and DPF field object, it can generate an enhanced image.
3. The enhanced image can be a TIFF file on disk, or a PIL Image object in memory.
4. The rendering and value pass processes are broken down by phase, and the essential RGB buffer, pick data buffer and variable data buffer are also generated independently.

Known problems:

1. Only support scalar data for 1 model and 1 field.
2. The JSON metadata part_name, colorby_var and pal_id entries can be further enhanced.

Potential next-step development:

1. Make it a class as the incoming requests increase. Currently, it is only a group of functions. But write it in a class can be long-term preferrable.
2. Grab as much information as possible from DPF inputs to improve usability. For instance, try and test if part_name can be retrieved from DPF model.
3. Refine the part-var linkage so that enhanced image can support a list of pairs.

[margalva]

@zhang-yuanrui I don't want to add dpf and vtk as requirements for pydynamicreporting. Please see how report_download_pdf.py handles a similar situation with PyQt5. If the module exists, then methods are defined. If it doesn't exist, then the module does nothing. I'd like to see something similar here.

I'd also like to see some tests added in the tests/ directory for these methods. You should add in the pyproject.toml the requirements for the extra modules only for tests (and maybe development), and create a few tests for these methods. Let me know if you need help here.