Support skipping values

[alugowski]

Support ability to read just the indices and ignore the values, i.e. pretend the file is a pattern file even if it isn't.

Example use case: unweighted graph algorithms.

Corner cases to think about:

• should the value be initialized to a fixed value or left untouched? Allow both?
• what should happen with array files? Both read into an array and read into sparse struct

[alugowski]

Implementation idea:

Add flag to read_options. Flag could be set automatically if the value type is pattern_placeholder_type, but should not require that.

Potentially make the flag have three states:

• read values normally
• skip values, but set the struct to a fixed value (pretend it's a pattern)
• skip values and don't touch the output struct at all

[galaxy001]

For the third state, I wonder how fmm treat zero values in matrix market file. Like 12 3 0.

Does fmm skip those lines?