Use consistent function and variable names for some matrix properties

[sjsprecious]

Matt clarified some terminologies for the matrix properties:

blocks = blocks along the block-diagonal matrix
number of blocks = number of grid cells
groups = sets of blocks, with each set containing a vector-dimension number of blocks

Then we realized that some function names may not be accurate. For example, this one means vector dimension times the number of non-zero elements per block. But it could also be misinterpreted as vector dimension times number of species times number of species (i.e., the full size of sparse matrix of a group). It is better to rename those functions and probably the related variables for better clarity and consistency.

Acceptance Criteria

• Use more clear and consistent function/variable names
• Pass all the unit tets

Ideas

• Use FlatGroupSize rather than GroupSize as function name?