Improve EphemKernel useability

[altairgomes]

Some tests should be made to ensure the readability and useability of the EphemKernel Class. These tests should include:

• Test if the kernel is available, is an allowed kernel and can determine the path from geocenter to the object.
• Determine and show to the user the limit of time the ephemeris can be calculated.
• Be able to define and save a default path where the kernels are stored.
• change code to spkid

[Bmorgado19]

Maybe it would be interesting to change the parameter code within the EphemKernel to spkid. This would avoid misconceptions about the word code.

[altairgomes]

We can also add a query to the SBDB to get the spkid automatically. This can be based on the following codelines suggested by @martinBanda .

from astroquery.jplsbdb import SBDB
sbdb = SBDB.query('1999 RC216')
obj = sbdb['object']
spkid = obj['spkid']
print(spkid)

[martinBanda]

Follows three ways to get the spk id:

1. Searching in the header of the bsp file.
2. Using the SBDB function from jplsbdb module (comment above by @altairgomes )
3. Using a API (process similar to item 2)

Being the item (1) the most efficient way (according to me), because it only depends on the existence of the bsp file avoiding query time.

I have developed the codes to get spk id using any of the three methods.