describe: provide more tag info about omega_h

[Angelyr]

I can start this soon.

[Angelyr]

I have the tool ready, but I need a clarification on printing number of entities:

1. Should I use std::cin for the input or arguments? How would std::cin work with mpi?
2. Ask for the rank or sum all the ranks?
3. Ask for dimension or sum all the dimensions?

[cwsmith]

Great. Thank you.

1. Let's assume for now the user would run describe once just to see what fields exist and then additional times to extract field specific data (for now the number of entities with a given value). For the additional runs they would append a tag name, dimension, and value to the command line arguments.
2. Sum across all ranks. We should only count 'owned' entities with the specified value.
3. Good question. The dimension should also be provided by the user on the command line.