Added EISCube.extract_points()

[MJWeberg]

Added a new EISCube.extract_points() method:

• Users can input a list, array, or SkyCoord object (see SunPy coordinate tutorial) containing the coordinates of any points inside the EIS field-of-view. Pixel indices are also accepted by inputting an array and setting "units = 'pixel'" (Astropy units work fine too). Input world coords must be in the helioprojective coordinate frame and an obstime at or near the time of the EIS observation.
• The .extract_points() method outputs a new EISCube with the data at the selected points (output shape of [1, N, n_wave]). Coordinate information about the selected points are stored in a new EISCube.meta['extracted'] dict. The method also updates .meta['date_obs'] and .meta['duration'] with the information at each point (rather than each raster position).
• The new EISCube can be fit and the results saved to disk, just like any other EISCube

Other minor changes:

• Slicing an EISCube will now update .meta['notes'] with a copy of the slice indices. This allows for better recordkeeping of data cutouts. This even works when slicing using real world coords and the .crop method, although only the array indices for the slice are noted.