Issues/54 add interval option to stypes.Counter

[lp6m]

• Add update interval option to stypes.Counter
  • add interval option argument to stypes._Accumulator and its derived class.
  • The expected value cannot be obtained by setting the update interval with nesting a counter with the enable argument of other counter.

    
    main = vthread.Stream(m, 'main', clk, rst)
    xcounter = main.Counter(initval=0, size=4) #0,1,2,3..
    ycounter = main.Counter(initval=1, enable=(xcounter == 3))#0,0,0,0,1,1,1,1..

Step: 0 1 2 3 4 5 6 7 x : x x 0 1 2 3 4 5 y : x x x x 0 0 0 1 #we want four zero!

The processing of `xcounter==3` takes 2 clock cycles, so counter _y_ is reset at _3rd clock cycle_, 2 cycles after the xcounter reset. And at _4th clock cycle_, `xcounter==3` holds, after two cycle, the value of _y_ counter is updated to 1.

This update can define counter with `interval` as following.
```python
cnt1 = strm.Counter()
cnt2 = strm.Counter(initval=1)
cnt3 = strm.Counter(initval=2, size=3)
cnt4 = strm.Counter(initval=3, interval=3)
cnt5 = strm.Counter(initval=4, interval=3, size=4)
cnt6 = strm.Counter(initval=4, step=2, interval=2)
step,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33
cnt1,x,x,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31
cnt2,x,x,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32
cnt3,x,x,2,3,4,2,3,4,2,3,4,2,3,4,2,3,4,2,3,4,2,3,4,2,3,4,2,3,4,2,3,4,2,3
cnt4,x,x,3,3,3,4,4,4,5,5,5,6,6,6,7,7,7,8,8,8,9,9,9,10,10,10,11,11,11,12,12,12,13,13
cnt5,x,x,4,4,4,5,5,5,6,6,6,7,7,7,4,4,4,5,5,5,6,6,6,7,7,7,4,4,4,5,5,5,6,6
cnt6,x,x,4,4,6,6,8,8,10,10,12,12,14,14,16,16,18,18,20,20,22,22,24,24,26,26,28,28,30,30,32,32,34,34

• Fixed the problem that the counter initial value is 1 larger than the expected value when size is not specified.
  • the initial counter value with no option arguments is changed to zero. This modification was applied to some tests. https://github.com/PyHDI/veriloggen/commit/c7a4d8a37b26be919acf9f4b238f9f741399517e

[shtaxxx]

Thank you very much for very nice suggestion and implementation!