BUG: replace reserved keyword freq with cadence

[jklenzing]

Description

Addresses #50

With the release of pysat 3.0.0, freq is now a reserved keyword. Changing to cadence for the purposes of this package. This is more descriptive, as the string is the desired measurement cadence.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

1. Import a module with a different cadence than the default ('1s') using the cadence keyword`.
2. Load the data
3. Check that data is at desired cadence

Example:

import pysat
import pysatMissions
test = pysat.Instrument(inst_module=pysatMissions.instruments.pysat_ephem, cadence='10s')
test.load(2019, 1)

Note that instrument registration is broken due to #49. This will be fixed in a separate pull.

Test Configuration:

• Mac OS X 10.15.7
• python 3.8.2

Checklist:

• Make sure you are merging into the develop (not main) branch
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• Add a note to CHANGELOG.md, summarizing the changes

[jklenzing]

NOTE: I've added a custom set of unit tests under the testing class to check that cadence works as expected. These tests should be run in addition to the standard inherited tests.

[aburrell]

Looks fine to me. I'd have used load_freq instead of cadence, but have no problem with the fix.