Add writing to Pulseq files

This required major reformatting of the Pulseq I/O system

Closes #8 (closed)

test/test_IO.jl

 @testset "test_IO.jl" begin
-    directory = joinpath(pwd(), "test", "example_pulseseq")
+    directory = joinpath(pwd(), "example_pulseseq")
     @testset "read v1.3.1 fiddisp.seq file" begin
         seq = read_sequence(joinpath(directory, "fiddisp_v131.seq"))