conda version without speasy support

[jgieseler]

solarmach uses speasy to obtain the measured solar wind speeds for selected spacecraft and falls back to an editable default value is this fails or speasy is not available. Because speasy is not available through conda, the conda version of solarmach skips its installation, and consequently, this version doesn't offer the functionality provided by speasy.

1. Add a mentioning of this to the documentation, so that a user might op to install the pypi version of solarmach, or pip-install speasy in their conda environment.
2. Investigate the option to automatically install the pypi version of speasy with the conda version of solarmach.