Change `Field.__repr__` to match representation in `pystencils.fields`

repr of Field should look like

y: uint16_t[246,704,558]
x(3): float[20,40]

to match representation when using pystencils.fields

E.g.

In [3]: pystencils.fields('x: float32[20,40]')                                                                                                                                                                                                                                
Out[3]: x: float[20,40])

In [4]: pystencils.fields('x(3): float32[20,40]')                                                                                                                                                                                                                             
Out[4]: x(3): float[20,40])