Move function `comp_ishape` to method of SHPC class.

Avoid the mistake eddy_index = 0.

Several scripts allow `ishape_last.txt` not to exist. For example
`eddy_dump.py`.

The code is adapted from `filter.py`.

For performance.

So we are reverting in part commit 26624352.

Especially, we want to allow missing layer `max_speed_contour` when
running script `plot_eddy_contours.py`.

The dates in the shapefiles should be the same for both orientations.

Because we want to move the logic on date out of the loop on
orientation.

So we are reverting in part commit d07eaadf. Because we want to move the
logic on date out of the loop on orientation. If we want to allow
missing `ishape_last`, we can do it in `open_shpc` or class SHPC.

The motivation is to manage an SHPC directory without opening first
all the files inside. The class allows to open files dynamically and
remember already opened files.

`slice` is a built-in class of Python.

A little clearer, a little less repeated code.

If `date < handler["d_init"]` then the result of the function is
silently wrong.

The global variable was used in the body of the function.

Adapt to the new definition of an SHPC.

Adapt to the new definition of an SHPC.