rename from->build for constructing from a indices block
implement support for extracting individual rows and columns
add utils for building zero and identity matrices
support building from StaticArrays
remove seperate spec helper
simplify merge type signature
add direct methods for row and column counts
force pointer access via #to_unsafe
fix issue with incorrectly dimensioned matrices following map operation
implement matrix addition and subtraction
implement equality checks
provide tests for all implemented methods