Provide option to show reference as area bw. 2 Parker spirals (instead of arrow/cone)

[jgieseler]

Similar to https://github.com/jgieseler/solarmach/issues/6, provide option to show reference as an shaded area between 2 Parker spirals.

[jgieseler]

With https://github.com/jgieseler/solarmach/commit/d2b0783ffdf7df7554cc9e8fd68d357dc8cae71d (upcoming release 0.1.6), SolarMACH.plot() has 3 new options for this purpose:

• long_sector: list of 2 numbers, optional Start and stop longitude of a shaded area; e.g. [350, 20] to get a cone from 350 to 20 degree longitude (for long_sector_vsw=None).
• long_sector_vsw: list of 2 numbers, optional Solar wind speed used to calculate Parker spirals (at start and stop longitude provided by long_sector) between which a reference cone should be drawn; e.g. [400, 400] to assume for both edges of the fill area a Parker spiral produced by solar wind speeds of 400 km/s. If None, instead of Parker spirals straight lines are used, i.e. a simple cone wil be plotted. By default None.
• long_sector_color: string, optional String defining the matplotlib color used for the shading defined by long_sector. By default 'red'.

1. Full example:

   
   from solarmach import SolarMACH
   body_list = ['STEREO-A', 'Earth', 'BepiColombo', 'PSP', 'Solar Orbiter', 'Mars']
   vsw_list = [400, 400, 400, 400, 400, 400]
   date = '2022-3-11 12:00:00'

sm = SolarMACH(date, body_list, vsw_list) sm.plot(numbered_markers=True, long_sector=[290,328], long_sector_vsw=[400,600], long_sector_color='cyan')

![issue32](https://user-images.githubusercontent.com/39489154/226619854-d3cd1846-5c1c-492e-8064-cdd721b81a10.png)

2. For `long_sector_vsw=None` (default), instead of Parker spirals straight lines are drawn:
``` python
from solarmach import SolarMACH
body_list = ['STEREO-A', 'Earth', 'BepiColombo', 'PSP', 'Solar Orbiter', 'Mars']
vsw_list = [400, 400, 400, 400, 400, 400]
date = '2022-3-11 12:00:00'

sm = SolarMACH(date, body_list, vsw_list)
sm.plot(numbered_markers=True, long_sector=[290,328], long_sector_vsw=None, long_sector_color='red')

3. Note that there still is a bug if the speeds in long_sector_vsw differ to some extent; then the plotting might not work as intended:

   
   from solarmach import SolarMACH
   body_list = ['STEREO-A', 'Earth', 'BepiColombo', 'PSP', 'Solar Orbiter', 'Mars']
   vsw_list = [400, 400, 400, 400, 400, 400]
   date = '2022-3-11 12:00:00'

sm = SolarMACH(date, body_list, vsw_list) sm.plot(numbered_markers=True, long_sector=[290,328], long_sector_vsw=[400,700], long_sector_color='cyan')

![issue32_bug](https://user-images.githubusercontent.com/39489154/226620344-a8d2db90-a6b1-480d-a2e0-6c17b0be11b8.png)

[jgieseler]

NB: Renamed fill_between to long_sector with https://github.com/jgieseler/solarmach/commit/f8ab303f8a95adf155c5683181b3a771dd45fe00